Noble outfitters has some nice belts for shows
You can also get inexpensive black belts from everday apparel stores that look amazing with Breeches. I have belts from American Eagle that work well with my breeches and look super classy in the show ring!
you could get a plain black c4 belt. i absolutely love leather black belts, although they're a bit more expensive. some great leather belt brands include tory leather & tailored sportsman