What is the stainless steel baseboard in a walk-in cooler/freezer?
It looks like galvanized steel to me. It  probably came with the walk-in panels from the manufacturer. 

You can get small pieces of galvanize steel at your home center,  I don’t know what application you are going to use it for but you could try looking in the roofing flashing department of the store for longer pieces. 

Or  you can get any size  and Gauge you like at a steel supply house.