Keep trying the little (non Big O tires or WalMart) tire stores.
I went out during lunch today and got 30-35lbs free at one shop and paid $50 for 2 5gal buckets full to the top plus a little. I'll go back again after next payday and get some more.
My personal experience is that a full 5gal bucket weighs around 130 - 140lbs and gives about 110 - 120lbs of "ingots". Now my ingots are soda pop cans with the top cut off using a can opener and when full weigh about 8.5-9lbs each. Sort of a one time use ingot mold. YMMV.
Grendel