Does USCIS have an approved list for education credential evaluation firms?

[ncjeffnc]

I got a I-140 education equivalency RFE. I searched the web and found many evaluation firms out there. Does USCIS have an approved list for firms doing this, like the list of physicians for medical examination? Which firm is the best?
Which one you used? Do I need to get 2 evaluation letters from 2 firms? Please share your experience.

Thanks!