Dance belts are measured by waist size. Although most dance stores don't carry an extensive selection of men's dancewear, most have a small selection of men's tights and dance belts stashed away somewhere. You'll probably have to ask the clerk where they hide the men's gear. You can also shop online if you live in a town too small to have a dance store, but be prepared for sizing problems until you figure out exactly what each manufacturer means when they say "medium".
(Dance clothing manufacturers have standardized their entire product line using female sizing, so even though dance belts are strictly for male dancers, that's how they are sized. Go up one size from your normal underwear size.)