You're probably looking for something that'll make a bright layer in damascus- if not, if you're just looking for a clean low carbon steel, call Aldo- he sent me a few chunks of very low carbon steel, like a 1006 a while back, he may have more. I kinda hate using "mild" (A36) because there's no telling what's in it.
Sorry, no clue where to get 203...hoping someone knows a source.