They are difficult to find. My wife freezes up and gets all self-conscious when I want her to mouth off.
If she's really aroused, and had a few glasses of wine, I can sometimes get her to talk a bit of filth to me.
I honestly don't think women realise the power they wield when they're potty-mouthed in the bedroom. The more demure and 'respectable' the woman is, the more impact it has.