Anyplace to find these locally, lowes Home Depot ,ace or just order from brownells
McMaster.com
If you are tapping into a receiver for scope bases, you will need a bottoming tap. The big box stores won't have them.
Just a wild guess what you are trying to do. 8X40 is a common thread for scope bases.
heli-coil it . that way you can change to 6-32 threads . coarser threads hold better .