Hi rmr,
I'm new here. Looking to learn about strap making. However, I have been collecting watches, specifically soviet era pieces for some years.
Here in the US, you may want to start with Otto Frei, www.ofrei.com
They have a great selection of watch parts and tools, just search buckles.
I would also recommend a vendor on ebay, goodcheapman. He has a very nice selection of buckles, deployment clasps, and straps. Delivery from Hong Kong (if I remember correctly), is surprisingly quick.
A 20mm buckle will measure 20mm as an inside dimension. If the width of your strap is 20mm, you need a 20mm buckle.
Good luck on your projects. Got any ridiculously cheap Vergez Blanchard pricking irons you want to unload?