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.
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?
Unless you're one of those guys... and nobody likes you guys anyway. You should still get a parts kit, though, so you're only mooching paint, and not o-rings as well.
All the basic o-rings you could need for your paintball gun