I havent had to buy any for our TT, but I have looked into this before. I have heard a lot of good things about the Denman tires and the sigma towmax tires. There may be some local dealers in your area that carry them, but if not, here is one site on the web to get them. Under the manufacturers list, you will see Denman, as well as Sigma, who carries the towmax.
What type of tire dealers do you have around there? Firestone, Bridgestone, etc.. I would check with them to see if they have these, or something similar.
The first place I would start would be the dealer that I bought the TT from. Every time I go in there, I see he's well stocked with both tires and rims. So try your local dealer. They would know best what type of tire to get you for the loads that will be applied to them. I know someone that bought some tires from Sears and the first road trip with the new tires one blew out. After changing that one the other blew out. He found out later that tire was really not suited for a trailer of that size. After almost killing the rep at Sears he now has proper tires on the trailer and everything is fine. Don't think that just any tire place will know the right information for your trailer. That's why I suggest going to a local dealer. They have all the specs (or at least they should have). Good luck