parts section of craigslist. I don't know about up there but around here these old chevys are everywhere. People always seem to be parting them out, especially the 2wd long bed ones because they're not worth much once the motor goes. If you can't find a 4wd cab you can make a 2wd cab work.
Cab corners and rockers can be replaced though, never seen one with a rusted firewall.
Also try fullsizechevy.com its a full size chevy forum.