Where can I affordable mountain bike wheels?

I have a mountain bike, with really messed up wheels. I want to get a new 26" set of wheels. Only problem is wherever I go online, they wheels are $400-1000. If I had that kind of money to blow, I'd just buy an entire new bike. Where can I find a set for under $100? I'm not looking to compete in any bike tour or anything lol. I just want to ride around casually so I don't need overpriced bike wheels.