That would be best. There are plenty of excellent freelance designers who will work on a per-project basis.
Depends on the designer.
But since it sounds like you don’t have those, you could also look on ThemeForest for designs you like, then contact the author privately. Many are willing to work on a freelance basis. Although you might want to target those that aren’t making that much revenue (why would they make themselves available for hire if they already make more selling on their own?).
Dribbble might also be a good place to look, but I’ve never hired anyone from there before.