Anyone know where to get ahold of a 3/8" stainless dovetail blank?

check brownells

Brownells has several slot blanks, but none in stainless. The Marbles is well finished.

http://www.brownells.com/aspx/NS/store/catsearch.aspx?k=slot+blank&ps=10&si=True

Bye
Jack

Go to a steel supply buy a small piece and get to work. Good luck, with a hack saw and a little filing you will get what you need.