I never saw more than 1 donor of my ethnic mix at any one agency, they were really hard to find! That was a big reason why I didn't want to be tied to one agency, because if the donor pulled out, I didn't have a 2nd choice at the same agency.
I didn't always have to sign a contract up front, one agency was very helpful when I stated my concerns and they said it was ok to pay the agency fee and sign the retainer AFTER the donor passed screening. Well, that donor seemed very enthusiastic at first, providing extra information that I requested etc, but when it was time for the medical screening appointment at the clinic she pulled out! Thank goodness I didn't sign with them, I did not lose even 1 cent, only some time.
The current agency I am with was tougher to negotiate with, but I did get them to reduce the non-refundable part of their fee and had them change the contract accordingly.
I already had the clinic chosen, and it took a couple of months of searching through agencies to get to my current donor. That includes the several false starts I had with donors pulling out, failing screening etc. Honestly I thought it would take alot longer and I was miserable during the search process, thinking it could be a year before I find someone. I started to be less specific about the ethnicity to broaden the criteria a bit, but then out of the blue as I was doing my usual database checks for new donors, I found someone with the same ethnic background as me.