Bought mine from GM. I'll have to scrounge around to find them - the same type of spiral shank rivet was used on the oil splash shield under the intake. You can pry them out of old intakes if you can find a couple - usually pretty good condition (and I would do like restorers do, don't use them under the intake when you rebuild one - might work loose. Drill and tap for a good capscrew replacement). The original rivets have spiral shanks, not threads, and just tap into place.
Regards -