So my only decent alternative was to make some from scratch. Using Fimo clay and some steel wire to make them a little bit more resilient, I made six of these, and although I'm not entirely happy with the result (they're far too bulky and chunky), they'll do the job:
When the buckles had cooled down from the oven, I gave them a quick layer of Citadel paint, Ironbreaker, drybrushed on to give a worn look.
While I had the clay out, I figured I might as well try to do the belt buckle as well, and it turned out a lot better:
That's it for now! I'll come back to you with a new update when the boot covers are ready!