Locally I buy it at a couple different places (most commonly from Calumet, but also EP Levine's, Hunts Photo, and Zeff Photo).
Online I've bought it from B&H, Adorama, Freestyle, Megaperls, and E-bay. J&C is gone now, but that was a great place to do business. Of the ones still around, E-bay is a great place to look for recently expired film and rare film, but you'll generally get no special deals for new film. Freestyle is just all around wonderful -- and you'll find some unusual films there like the whole Efke line, Arista films, orthochromatic films, etc that you can't get elsewhere. From B&H and Adorama the prices are basically standard, and you have to weigh the cost of shipping against the cost of sales tax if you buy it locally. But they have basically every standard film and more, esp at B&H.