FWIW, I have a source for 5mm key stock, but you have to cut it to length. Here's a link: M5 x 250 mm (10") Length Metric Square Keystock — Type A2 Stainless Steel
The outfit is Tacoma Screw, in Tacoma, WA. I buy a lot of hardware there. Fortunately, it's fairly local to me. I live in Gig Harbor, which is on the other side of the Narrows Bridge (think Galloping Gertie).
Gary