Where can I obtain a new strap for a Graphic? I just found a real clean 2x3 CG but the strap is rotten....

Occasionally nice straps show up on wePray, and sometimes Midwest Photo has new replacements I think.
Myself I use 1" nylon web doubled over and sewn just right to allow a snug comfortable wrap around my hand and a solidgrip on the camera. Though the originals should have the lower clip to allow detachment for ease of installing the camera on the side tripod mount, I can get mine on without the clip. I also recently tried a soft leather wrap on a strap for a Crown 45 in which I did use the clip for the lower strap lug and it is really comfy.
Of course a person could also make a new leather strap, esspecially if they had all the goodies like the snaps and stuff. Crafty.

As TM wrote, MPEX sometimes has straps. Also, someone is selling straps on eBay. They're not offered continuously, so if you don't find any first try you'll have to keep looking.

I find the clip that allows one end of the strap to be detached from the camera invaluable. Otherwise, setting up to shooti verticals from tripod takes way too long. So keep the clip from your rotten strap.

As troublemaker said, 1" nylon strapping, available from the fabric store, makes a very serviceable and good-looking handle. This woven material comes in several colors, but black is the obvious choice. I made a handle for my Century, with nylon buckles from (IIRC) a Tandy leather store. You can sew the strap, or you can melt and fuse it with a low-wattage (25W) soldering iron, which I did. Either way makes a permanent connection.