I didn't mean to be blunt thought it was a genuine question, really depends what u want to work on, like thin... Car body work or welding 10mm plate together?
Get some mild steel flat bar 25mm by 3mm costs £6 a length for 6m it's a good start and cheap, prep it with a flap disk and good for doing t sections aswell
As suggested above, something like 4mm-6mm mild steel plate or bar. I found an old garage roof truss in my garden, which has provided me with about 8m of 40mm x 6mm mild steel. Just needed cutting down to give me a few manageable chunks (I cut off half a dozen 15cm bits), then a flap wheel to remove the paint and rust.
Either buy it, or have an ask around to see if anyone's got any scrap they wouldn't mind donating.
The important thing is for it to be plain, uncoated mild steel, and over 4mm thick so it'll take as much heat as you like until you gain control over the arc.