Hi ScottI don't know of any good Pitts' for sale, but I'm sure there are a few out there. Just make sure you get a very good pre-buy inspection before you buy one, and from an experienced Pitts mechanic.
There are advantages to both the S and the T. The T has more performance than the T. The constant speed prop actually stores energy and gives you better performance, although if you get a T I'd recommend you go to an MT rather than a metal prop. The metal props just aren't good for the crankshafts when you put any kind of torquing load on them.
There are also some excellent S's out there. The choice really depends on what you plan to do in competition aerobatics, how far you want to go, what your goals are etc.
Also, don't discount a "homebuilt" Pitts as opposed to a factory-built Pitts. There are some fabulous Homebuilts out there, as good or better than the factory versions.
Not much help, but hopefully a little.
Patty