Basic Supplies

There are some basic supplies that almost every Nerf club will need. This is pretty basic stuff but provides some insight into things to consider.

Foam Blasters

Orange tips on blasters is always recommended although if you are on private property you might not need them but should definitely have them for public property. Personally I like having orange tips on my blasters.
One thing to consider is whether you are going to limit the type of blasters people are allowed to use.
1. I would say most clubs don't limit the type of blasters that people use (other than FPS limits) but that doesn't mean you can't. Some people might want to have a Rival only league or focus on a particular type of blaster. Groups can also have specific games where they limit the blasters, a great example of this is pistol only rounds which can be fun because that means a person who doesn't have a lot of money to spend can still play on equal grounds with everyone else.
2. If there is a specific style of play that you want then this is one mechanism for creating that atmosphere but remember by its nature it's going to limit who will participate. One thing you can do is create an agenda for your event which tells when certain styles of games will be played and what the requirements are to participate.
Obviously you'll need at least one blaster for yourself.
1. To be honest, it probably won't stop at just one.
A few loaner blasters to help get people interested is always a good idea.
1. It's important that these be somewhat decent so that people have a good time. The loaners don't have to be expensive, for example, the Kronos is an inexpensive blaster and while it doesn't have a large ammo capacity it is one of the more accurate Rival blasters which makes it a great loaner blaster for our Nerf club. For those higher FPS games, Dart Zone has the Aeon and Nexus Pro which are likely to become the go-to loaner blasters. Hopefully, you can find an inexpensive loaner blaster that matches your groups style of gameplay.
2. It can be nice to have some loaner blasters that are a little overpowered as well but not necessary. On occasion I let people use one of my Nemesis blasters which they always get excited about especially the younger kids, however, the Kronos is the real workhorse of our club.
Having some specialty blasters for certain game modes can be a lot of fun but isn't necessary when just starting out.
1. For example, I purchased 4 Nemesis blasters which turned out to be great for creating bunker defence game modes or setting them up on a turret for people to capture and use during a game.
2. Other specialty blasters include things like rocket launchers. Captain Xavier has a great game mode called Titan Ball which utilizes 3 Titan blasters.
3. Really anything can be a speciality blaster as long as it's unique and offers some interesting features.

Extra Ammo

Lots and lots of ammo. Nerf wars always tend to be more fun when you don't have to frequently clean up ammo. This can be accomplished by just having a ton of ammo, placing dart limits on players or limiting players blaster capacity. I would say the best of the three options is to have just a ton of ammo. Since my club is focused on Rival style blasters it was difficult to supply sufficient ammo when we were first starting, especially since we had several full-auto blasters early on. We mitigated this initially by limiting the use of full-auto blasters in our games but eventually, I ended up purchasing enough ammo that this became less of a concern. I recommend having people donate ammo rather than trying to split it up at the end but it's not that big of a deal as long as there is the recognition that some ammo will get lost or damaged.
Communal dart bin - A lot of groups have a communal dart bin that people have donated too. Usually, it's pretty easy to build up a communal bin of full-length darts since they are inexpensive and many people will just leave whatever darts they bring. I'm actually not certain about whether people tend to leave half-length darts or not. With Dart Zones recent move to sell half-length darts at Walmart this might become more commonplace. Rival style rounds can get expensive to supply but they do tend to last longer than other types of darts. The communal bin can get reused at future wars. It's a good idea to sort through the ammo in between the events in order to pull out any damaged ammo.
Some players may use some speciality ammo which typically they would need to provide for themselves.

Bunkers

This will depend on the field you are playing at and your game style on whether you need to bring more bunkers to supplement the cover that is already there.
If you are using a building like a church or a school you may be able to use tables or chairs to create bunkers.
Other common materials for bunkers include cardboard boxes, wood pallets, plastic bins, etc. One of the great things about nerf wars is that you can use almost anything as a bunker.
If your looking for bunkers that are transportable a lot of people will build frames out of PVC pipe and tarps/ cloth. These are nice because they can be disassembled and rolled up for easy transportation and storage. One thing you will need to figure out is how to stake down the bunker or prevent it from falling over.

Printed Rules

Hopefully, this website helps with regards to printable rules. It's nice to have the rules printed to remind you of the details when explaining them but also to resolve any possible arguments.
You can always just have them in electronic format as well.
This becomes most helpful if you play a bunch of different style games or are creating a new game mode.

Team Indicators

I would call this optional supplies depending on the size and regularity of your group. If you have a small group and people that regularly play together you can probably get away with not having team indicators. The team indicators are really most helpful for large groups or new players who don't know anyone.
These can really be anything that easily distinguishes the two groups. Bandannas, cloth strips, flag football flags, Rival team indicators, etc.
The important thing here is to be consistent and make sure they are easily identifiable.

Extra Eye Protection

Eye protection is essential. I know some groups choose not to use it but my personal opinion is that it should be required. It's just not worth the risk of damaging your's or someone else's eyes.
I strongly suggest having spare eye protection for people to use. It's not that expensive to buy a pack online. Some groups will have extra eye protection which is sold to those who forget to bring their own.

Water

Having some source of water for people is always a good idea. It can just be a big water cooler, drinking fountain or bottled water (maybe for sale).
You can always ask people to bring their own but it's nice to have a free source. I would at least have some water in case of an emergency.
Stay hydrated.

First Aid Kit

A simple first aid kit is always worth having incase of minor injuries although people should always seek professional help if needed.