What is a really good website to get sold out concert tickets from?

On May 5th I really wanna see All Time Low, Hey Monday, and Yellowcards, and their not selling tix at the door. My friend says that there are alot of websites that sell seold out concert tix for good prices. Bu the thing is, I have prom coming up so thats the only challenge. I REALLY want to go but I wanna pay a reasonable price for it cause some of the sites that DO sell them cost in the 100s. Any good suggestions??