For costumes, you can just Google it. For example, "Kyle south park costume" and a bunch would come up. There was also a Cartman mask in my local Halloween store this year.
But for actual outfits like the boys wear, you could easily do that. It's not that hard to find an orange parka, for example, so you could easily be like Kenny.
For random clothing, you could try zazzle.com and type in South Park. They have tons of South Park shirts. I also bought a pack of 5-pack South Park female panties at Walmart (haha, but I'm a girl and I don't know what you are!) and I saw a Kenny hat and a Cartman hat at Spencer's (or Hot Topic? I don't know which.)
(Oh, and there were plenty of South Park boxers at Walmart too.)
