Not clear if it's an agency per se, but a half-dozen or so providers listed on p411 use them for screening and booking, at least for new clients.

I booked time with Heather North a few weeks back.  Since I already had 3 oks on p411, the screening was pretty straightforward.

Bay Area Support Services has been around for awhile.  The owner keeps a low profile but posted once in awhile on the now defunct board.  I did see a negative post on another board by someone who alleged to have been recruited by BASS, but overall it has a good reputation.  The ladies ads all have very similar format, but they all have good reputations.