You might look at the Burris website or see if you can get a list of their dealers. I got my Speed Bead from Optics Planet, though they sold a number of other Burris products as well, though I'm not sure if they sell guards. I remember seeing them on one site, but cannot say which it was right now. I'll bump around and see if I can help point you in the right direction.
Luck Counts, good or bad