You definitely need a scope that's made for air rifles. Any of the big online suppliers carries them. I'd recommend Air Gun Depot -- bought both of my airguns from them, and their service was excellent. PyramydAir has a larger selection of inexpensive scopes (starting at $10
), but you pretty much get what you pay for, and I'd trust AGD not to be selling anything that's absolute junk -- theirs start around $35. If you don't want to spend at least that much, you might as well stick with the sights your rifle came with...
http://www.airgundepot.com/airgunscopes.html#paging
http://www.pyramydair.com/a/Accessor...F10CC021.app03