[EC900]

You can try MetalSupermarket (if they’re in your area) for new steel sheet cut to size. If your mod will require bending and cuts of course this becomes more searching.
What I wanted to do is learn sheet metal welding, friend had a body shop that always saved some scrap panels for patching. Sheet metal shops or hvac shops are plenty resourceful too for cuts and bends.
For just welding, check a local body shop and ask to rummage for some scrap, which they also save steel for metal scrappers to recycle, or even a junkyard…bring along some cash, an 18v 4” grinder w/cutoff wheel and some eye protection.