The basics are mostly that stainless steel is too expensive and doesn't have the thermal properties you want anyway, and that alloys that have chromium in them are probably not the best idea because of the toxicity of chromium, even though it probably won't put any in the food, and these alloys are also expensive. It's unlikely that someone who deals in metals professionally will accidentally sell you a chromium alloy. They know they can get a premium for it from people who need it for reasons.
It wouldn't be unreasonable to just explain to the vendor what a pizza steel is and ask for the cheapest piece with the dimensions you want. At least 3/16", not more than 1/2". It will probably be A36, and if you're lucky they are willing to make you a deal on an off-cut remnant. My 14x14x.5 slab was $40 including sales tax.
I have a hard time believing that most people in civilization are more than an hour from an outfit that sells steel sheet, angle, etc. You'd have to be somewhere pretty remote. For me the primary vendor is MetalMart, about 15 minutes away. But they are hardly the only one.
