Piece of advice, the agencies charging 8+ % is not the norm but you might see it on here a lot.
Feel them out, and ask what actually covers that "fee" besides ad-spend.
You can also go your own route by purchasing old, aged FB accounts using an anti-detect browser with your own cards for the cashback, if that is an option for you.
Discuss what you plan on running first, and even better if the agency has experience with your niche.
Best of luck.
Update: Just noticed now this is from last year, did you end up finding good accounts?