Your local plumbing contractors - particularly those that specialize in commercial new construction. They buy solder, cases at a time, and may be willing to make a few bucks by passing on their bulk discount to you.
Also, radiator shops. Their drippings and trimmings are sold for scrap. If you beat the scrap price (or even just keep them from having to drive to the scrap yard), they'll usually make you a good deal.
I get mine from a commercial plumber that specializes in hospital and school remodels, and commercial new construction. (Residential plumbers don't use much solder, these days.)