From a slave's perspective, finding a mistress can be very tricky online, which is how I've tried. I spent quite a bit of time on InformedConsent, a UK-based BDSM site, and often searched through the Scotland personals. There were a sea of male submissives looking for dominant females, while there were only a handful of female doms, and with most of them I didn't fit their specified requirements.
I got the sense all these women were bombarded with pestering, poorly-written messages from wannabe slaves, and that soured them on the process as a whole. I don't know if that's the best environment for starting a relationship, to be honest, though maybe I just had a bad experience with it.