You could try:
SARCO
Numrich (egunparts.com)
Poppert's
Shop around-the issue will be $20 order minimum. I have a Walther P38 circa 1944, and the locking block broke a few years ago. I ordered a new one from Numrich and got one which turned out to be a WWII part b/c it had waffenampts on it. I was very surprised.
Good luck.