I'm not a swinger, but as a psychologist I have some familiarity with the issue. i think you are right that the number of women looking for this type of relationship, especially with a stranger, will statistically be very low. Most women get into these relationship through their steady relationship, and usually with friends. I doubt there are very many women out there actively searching for this type of relationship. If you find one who is, be sure you're not dealing with a prostitute or some kind of set-up. Let's face it, women have no trouble finding men to do just about anything sexually, so I would be very skeptical of anyone you meet through a website or other medium.
Be careful.