I'm another one who's used The Chemistry Store -- in fact, I just got a 10-pound pail of potassium carbonate from them. There is one caveat about them, though: They don't tell you your shipping costs up front (you've got to give them your credit card number first), and they charge a $3.50 flat handling fee per order. For instance, my aforementioned 10-pound pail of potassium carbonate cost $14.63, but shipping was $12.19 and with the $3.50 handling fee it totalled $30.32. That's still fairly cheap; the chemical alone, without shipping, would have been $33.00 from Art Craft -- but OTOH, if I'd bought from Art Craft, I would only have bought 1 pound or perhaps 5 pounds, so I'd probably have spent less. (TCS only sells potassium carbonate in 10- and 24-pound pails.) As it is, I now have a lifetime supply of potassium carbonate sitting in my darkroom.
Overall, it's a pity that TCS doesn't have a slightly wider selection. I often buy elsewhere just because I need two items, but TCS doesn't have one of them and I can't justify the $3.50 handling fee for just one.