Your bike should still be under warranty. And if you bought it in May, the store should have a record of your purchase so you shouldn't need any kind of warranty paperwork. Because your wheel is a Bontrager, I assume your bike is a Trek. If so, you're in luck. Trek is very good about honoring their warranties. Take it back to the store and see what they have to say.

If for some reason you need to buy a new wheel, you can buy a rear wheel by itself. Hopefully you won't have to.