When I was building my '82 cb750F, I went to Zedno cycle for a lot of parts. Great cycle salvage yard, prices are fair. Only problem I had was they are in Kitchener. I would make a list of parts I needed and call ahead to see what they had. They would always call me back within a couple days and let me know what they had and how much. 99% of the time they had what I needed, then I would make the drive out there.
F.S. Motorcycle is a gta shop that was also a great help. The guys name is Glen. Very helpful. Sells mostly aftermarket parts for vintage bikes.
I would get new parts from
www.vintagecb750.com (which doesnt really help you). I haven looked but maybe there is a GS site that carries parts.
Also try
http://www.partsnmore.com/ Tons of stuff for Japanese motorcycles.
Good luck with the build.