Um, any Sears, auto parts store or department, JC Whitney mail order, etc.
Think of it as a big metal tongue depressor; the ones I have are about 1 inch wide, 10 inches long and 3/16" thick, aluminum alloy. The workig end is slightly flattened to better wiggle between the tire and rim.
There's another tire tool that's also very useful: It's a metal rod about 1 inch diameter with a bite taken out of the side near one end. After you open a gap with the tire iron(s), you insert this, with the notch facing the rim so it doesn't slip out. You bang the side of this with a hammer to scoot it around the rim, working the tire off the rim.
Here's a link to whitney's offerings, so you can get an idea: