A belt's a belt and a buckle's a buckle. If you wear it with your kilt, it becomes a kilt belt or kilt buckle.

Generally kilt belts (which are properly called dirk belts) are considerably wider than those worn with trousers, and the buckle is distinguished by some sort of Scottish motif (thistles, rampant lions, etc.). By and large the dirk belt looks "right" to most people when worn with the kilt while thinner belts, especially for larger size guys, don't work as well.

Given the cost of dirk belts, you would probably come out ahead if you have something made by a local craftsman that would allow you to use your favourite buckle. Just make sure that he understands you want the buckle to be easily removable.