I have been searching and trying to find OEM bolts for my truck…Im trying to find bolts to attach the rear bumper to the frame AND bolts to attach the truck bed to the frame and I have tried to search at various GM parts departments and online to no luck. Do anyone know where I could look to find bolts? I am rebuilding my truck from the floor up to the top. Thanks!
Try Classic Chevrolets,you will have to 'google" around for their web site.
If the fasteners don’t have GM part numbers, they were probably standard SAE items installed at the factory. You should be able to find suitable replacements almost anywhere…Google “nuts and bolts”
I’ve done quite a few full restorations and the first step for me is to buy the OEM assembly manual(s). These reference books show every assembly in great detail and also list the OEM part numbers. Armed with the part numbers, I have been able to purchase just about any NOS fastener from bolts to clips to cage nuts right from the GM parts counter.
These reference materials are not cheap (~$150) but if you’re doing a full restoration over a long period, they are also invaluable when you can’t recall how something went back together 
The parts guys are not likely to have those reference materials on hand anymore so you’re going to have to supply them with the part numbers. If you can’t or don’t want to buy the manuals, you might try a make/model specific board and plead your case for someone to look up the part numbers for you.
For restorations where originality wasn’t a concern for me, I camped out at the local hardware store and matched up what I needed.
The service manual, ST 375-91, doesn’t show part numbers. Torque, location, and quan only. Good luck. My 91, 4.3, goes at altitude for 75 miles each day and runs great.
That is why you need the ASSEMBLY manual(s).