You can get hammers at your local Lowes or Home Depot, but I think the cheapest place (and biggest variety) I've found is Harbor Freight Tools. Last time I stopped in, they had 2# increments from 8 - 16#, all for around $20 or$24.
The tire place we go to is great. We offered the guy $20 for his trouble, but he wouldn't even take it. Apparently it's pretty spendy to pay someone to come out and haul them away for recycling, so they're glad when someone takes a few off their hands.