I need a rear sight...

[Fyrtwuck]

Put it a vise, file to the desired width, treat it with Cold Blue and install.

Have you tried contacting Walther?

 

[druryj]

Earl's is a good bet to have the part you're looking for. He somehow got this domain name, but it's known as Earls amongst the Walther Crowd. Prepare to pay though. Earl ain't cheap. Here's the link:

http://carlwalther.com/

 

[ldp4570]

It will only accommodate a military sized sight.

That's what files are for!!