I don't really know of anywhere specific. If you shop around i'm sure you can find something nice. I haven't heard of to many guys that were unhappy with thier tubes, even if it was a cheap one. So if you see a tube that looks good and in your price range, I would say to go for it. I fish out of a u shaped tube, but I had a round tube for years and was very happy with it. Good luck.-Phil
Right now you can find the Creek Company Round boat at about $45 if you check in the right spots.....I just bought mine from work for $40 even. Try out Cabelas, i think they come with free fins...
rei a sporting goods store has the caddis high sierra tubes for around 45$, but only in teal and black and those colors are hard to find at most stores, you can get them fromt the web site for about 55$ after tax and shipping.