The GM dealer will sell any bolt that is on their diagram. I've had to get brake caliper mounting braket bolts and wheel hub assembly bolt both from the dealer. Both I couldn't find at the hardware store or even a junk yard. Also if the grade is 10.9, much harder to find in the hardware or auto stores.
I went to a junk yard and asked if they had a container of random bolts for me to match up. They told me to look on the ground, that's their container of bolts.