I was wondering what to wear with my kilt casually, as the belt I had was WAY too formal. I dug through my old gear and came up with the Sam Brown belt (law enforcement type) I had from when I was a deputy. Turns out it's the perfect width, and the basket-weave pattern looks great.
Try finding one in a pawnshop somewhere.

I also sometimes wear a military web-belt with my camo kilt.