Several options:
Stillwater (as mentioned above)

Cavscout: first quality handmade by X Marker in Georgia (US)
http://www.oconeeleatherworks.com/MiscAcces.htm

If on mini-budget:
Belt from thrift store. Cut off end (oppossite one from buckle) and add neccessary holes. Either get a belt with the appropriate buckle or get the dritz stamped buckle from a craft or fabric store (less than $1.50). A Bit more work and not as nice as Cavscout's but cheap (about $2.50 for one I did on one of my earlier kilts).

As for me, I plan to use Cavscout's for my next few kilts (at least), as well as retrofit an older one or 2.