Cowboy shooting is very active in New England, with matches somewhere virtually every weekend, here is a list: http://www.northeastcas.com Other useful links: http://marauder.homestead.com/irons.html and http://sassnet.com I'll point that the SASS Wire is NOT SASS, like most internet boards people are a lot ruder then in person, but the rules and other manuals are worth looking at and you can often get good advice in the forums.
I've been doing it for more the 17 years and I'll offer some advice: If you choose to start cowboy shooting think long and hard about how you want to shoot it now and in the future, this is not an inexpensive shooting sport and buying say guns in 45 Colt and then deciding a year in you want to go fast and be competitive and having to switch to 38 Special is going to cost a lot of money. Do your research before rushing out to buy stuff.
I don't think any gunstore caters specifically to cowboy shooters, but many should have at least a small selection of suitable firearms.
Despite all the caveats I listed above, I find cowboy shooting to be great fun with mostly great people or else I wouldn't still be doing it after all this time.
The Tri-state championship were just held at Harvard last week. I was there on friday early afternoon and turnout looked good, plenty of cowboys (and girls) there. I dont' know of any shops that cater specifically to SASS people though.