I never had a problem finding gay ladies to date and marry in Fable III (before I scratched up my copy of the game when my xbox fell over, d'oh!). There were times when I had as many as four wives (and several other ladies on the back burner, ready to step in). I even had two in the same town, once. (That didn't go very well once they found out about each other.)
I generally found them just by pretty much wandering up to random ladies in the town or area I was in and talking to them so I could see whether or not I could get them to date me. (Similar to my real-life strategy.) I think the gay/straight ratio in most areas is less than 50/50, but not by much. 35/65 or even 40/60, maybe. This is all just speculation on my part, and speculation based on memories of a game I haven't played in at least eight months. But that's how I remember the game.
Update: I've been playing through again. The ratio is drastically lower than I remembered, but there seems to be at least one or two lesbians in ever town. The number of gay men seems to be the same.
'Nother update: A very easy way to find lesbians crops up if you've finished the game as a good guy with enough money to save most or all of the citizens of Albion. Just about everybody you meet loves you if you finish the game that way, so if you bump into a lady and the popup says she's your Best Friend, that means she's straight. If you bump into a lady and the popup says she's your Love, that means she's gay or bisexual. Vice versa for bumping into men around town. This still requires you to check each individual for their preference, but at least now you can do it without Interacting with them.