Ive never seen a ss folder parts kit. I bought a ss full size rifle kit for $900-950 a year or so ago. Someone had a bunch of pre or post samples that they cut up. I even got to pick from which of the 5-6 guns I wanted to have cut.
You may find a blued folder kit. But odds are you will not find a ss folder kit
Ive seen them listed a few times on sturm. Give it a year. Something will show up
You probably won't find a folder kit, because they sell the stocks separately for big money. If you don't need the stock, buy a rifle length kit for parts, and have your barrel cut down to 13". Gun Broker seems to have kits regularly, just keep looking daily, and a Stainless kit will pop up.