I've heard the argument for doing a tailwheel conversion straight in a Pitts but I personally think it is a rare pilot that would be able to do that without costing himself more in terms of time, money and possibly confidence. I think you'd be better off doing some tailwheel hours in something like a Cub or Citabria and really, really get your landing abilities sorted before jumping into a Pitts. Get something like 20 hours or so and get yourself to the point where you can really see the glide slope and anticipate the touchdown point, and be able to land in a variety of conditions. Whilst you could do your conversion in a Pitts, everything happens so much faster and much more extremely, that for the first few trips your head won't be able to keep up with what the airoplane is doing. You'll have that experience with nearly any taildragger, so you might as well have it first in something a lot slower and more benign.
Then any of the places mentioned will be as good as another. Alan Cassidy is known for his aerobatic instruction, and some time spent with him for competition training may be well placed, but I don't know that you'd need to go to him specifically just to convert to a Pitts if he isn't conveniently local.
That's just my .02 worth, but the world expert on landing Pitts is Budd Davison. In fact, that's all he teaches these days. Drop him an e-mail and ask him his advice. He's always been very forthcoming when I've e-mailed him questions.
Good luck and have fun in any case. The Pitts is a great machine and you'll never regret getting to know it!
Pitts2112