The buckle tabs don't have to be tartan. On military kilts, the buckle tab is often solid black material. In any case, it doesn't matter because the strap will cover the tab when worn. So if you can't spare the extra tartan material (culled from some discreet area of the kilt), don't worry about it. Personally, I would just make them out of black material rather rip fabric from the under apron.