It’s very hard, it requires a lot of searching until you find something with no results, and then figure out if you want to do it or not.
The trick is to do something original, even within a crowded category. Sometimes you see logo designers that specialize in restaurants, so you have to search “restaurant logo” to find them. Same goes for religious book covers, romance book covers, etc. For voice talent, I imagine there must be a few you can find if you search “Spanish accent.” So if you need someone that sounds like Ricardo Montalban, that’s how you find him.