I don't know if this will help or not, but you could check with a dance studio.Seems like they always have a western dance at every recital I've been to. To buy them new would be pricey and probably not available in a week, but they might know of someone who would sell their costume cheap.Or do you know of any families that have children in dance? They might be able to help find one...
Otherwise, Target/walmart may have kids cowboy boots.With jeans,flannel/plaid shirt,bandana,cowboy/straw hat-should be a winner!