Today I went to three different places (REI, Joanne's and Michael's) to look for buckles that would work with this thing. All three had them, but they would not lock into place into the "female" buckles on the bag. They would slide in, but just not lock into place.
I tried 3/4" - which seemed to be the right size, as they fit snugly. But they would not lock in. So either Givi is using some oddball buckle, or maybe they are using a "metric" buckle that is just enough different that an "imperial" one will not work.