http://www.BudgetCastingSupply.com/ lots of good stuff here. Note that their prices include shipping in the USA.
In Southern California: Independent Foundry Supply. This place is the cookie jar for hobby foundry guys. They sell to the industry but happily sell to walk-ins for the same prices! Ask for Cameron, the owner's son - he runs the place. They have crucibles from teacup to jacuzzi sizes, many kinds of sand, clays, sodium silicate, parting sands... you name it, they have it and it's top quality and in-stock. I live 90 miles south and it was WELL worth the drive.
Also Lancaster foundry supply in PA does sell direct to the public and will ship by truck.
http://www.axner.com/ Good source for Sodium Silicate for cores.
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This is not a link because I already get plenty of spam!