They sell damn near any alloy you would ask for, but I typically buy: (92% Tin, 7.5% Antimony, 5% Copper or 92% Tin, 7% Antimony, 1% Copper)
Hallmark is in Rhode Island. They sell 7# (nominal) bars, but the exact weight will vary. Again, they charge you based on the actual weight sent. They seem willing to supply small orders. Though I typically order 10-20 bars, I once ordered only 5 bars. I've no clue if they will sell less.
More often than not these guys have seemed to have the best prices - at least for the quantities I buy.