Yea, there are a few sharks that would fit in a 125. I know the Marbled, Coral Catshark, and Epaulette shark would all be ok, but you should ask risc he would know better then I. You're probably going to want to stay away from live rock or use little if your putting a shark in there so it will have as much room as possible. But you should put something in there that the shark can hide in during the day time. My banded catshark hides in the little bit of live rock that I have during the day. You're definitly going to want a protein skimmer and a big wet/dry. You can read risc's post on filtration for sharks. That should help you out.
I use to live in Parker. That's where I went to high school. Ponderosa, I don't know if you've ever heard of it. But now I live up in Golden. I play football up here at the Colorado School of Mines. I just wanted to know if you know any good fish stores. I've only been to three. Sherman's Tank Company, Neptunes Tropical fish, and The Reef Gallery. The only one I like is Sherman's. It's crowded inside but it's cheap and the fish are always healthy. He'll aslo order anything for you, at a good price. You might want to go in there, and ask him about a shark. Reef Gallery is way over priced and Neptunes just isn't all that good. Do you know any good ones?