How to approach? Any bisexuals?

So being bisexual and being in a relationship could either be a good or bad thing, for my situation I'm with a man whom I've Been with for quite some time and our sex life has always been amazing. But I cant seem to shake off wanting to have relations with females. (I know you can't shake off liking a gender forever but in the bedroom it creeps into mind) Cheating is a no go, obvs, but we're both OK to have a threesome. My question here is, how in the hell do I find a woman who's going be down for that? How would I even go about approaching the woman?