There is always some at the market hall shows. Some of the ammo vendor guys have them under their tables, so sometimes you just have to ask. Other guys will have a shelf or pallet of them.
Last ones I bought were at market hall, 5 bucks for some 30-cal cans in almost new condition.
Haven't been to a mesquite show in a while, but it's usually all the same vendors, so odds are good that you could find them there fairly easily.
Cheaper than Dirt in Ft Worth also usually has some. Bought a few 50-cal cans from them... 12 bucks each.
