What size screw and length are the screws that come with the Burris fast fire and where can I find a pair?
6-48 x .375" with a quick search. I'd try my local Ace hardware first, they carry gun screws in the specialty screw section where I live.
https://www.burrisoptics.com/sites/default/files/content/products/manuals/fastfire3userguiderev10.2.15.pdf