I am looking for a source for a #9 viewfinder mask to use with a 210mm lens on a 4"x5" Pacemaker Speed Graphic. Any suggestions?

I assume that you've tried the auction site? These masks (in various sizes) seem to come up quite often. Good luck!

I also need to add a focusing scale for the 210mm lens and figure out how to switch the side mounted range finder back and forth from 135mm to 210mm.

Side rangefinder Kalart will only adjust to 165mm focal length. A hand held or hot shoe rangefinder could be used to get the subject distance then transfer that distance to the focus scale.

I got a NOS Singer Graflex Sorts Viewfinder Mask, which is a plastic insert that fits in the sports viewfinder.