We can't count how many times we've said this, but we'll say it again. You need to get a parts kit for your paintball gun, and there's only one way to get out of that statement - if your buddy is getting a big one. But in this case, you're the one getting the big one, and this is the only way your buddies can be off the hook for having their own (be sure to charge them, per o-ring, of course).
Let's face it, seals bust and o-rings break entirely too often for you to be caught at the field with your proverbial pants down. It's just as basic as buying paintballs - you wouldn't show up to the field without paintballs would you?
All the basic o-rings you could need for your paintball gun
Extra screws and more obscure seals and o-rings that you may not normally need... but will definitely come in handy when that time comes.