I'm in western CT and buy from McEnroe's organic farm in Armenia NY if I need anything—their compost is great. Most of the time, though, I mix my own potting soil, using homemade compost (if I have it), vermiculite or perlite, peat moss, and sieved garden soil. It can be tedious to do, but it's worth the trouble, IMO.
Now that the season's over, the used stuff goes into the flower beds.