Try looking in a shop that makes holsters, particularly if they make police belts. They are frequently wider than other belts. Sometimes they even keep them in stock. The belts from motorcycle jackets also make great kilt belts and they are usually much heavier than the kilt belts I've seen in some shops. Don't overlook tool belts, some times they even come in a cool basket weave and you'd just have too re-dye them darker.

Jamie