~grins at Marcus~
He's right... there would be an almighty Battle of The Unicorns if you found this woman.
The reason these elusive women are quite rare is that the idea of a Unicorn is based on the idea that bisexuality means "equally attracted to men and women". Sexuality is a sliding scale for the majority of people. I have trouble finding one female secondary that I am genuinely attracted to; let alone finding two of them, who happen to be a couple already.
What is it that you are looking for? You say long-term and committed - would you like a triad? Would all of you be on the same level, such as primaries? Would you want her to eventually live with you? If you want a long-term thing, but with a more secondary kind of setup, would you want it to be a closed-poly relationship? Or would you be fine with her dating other people?
I do see the merit in sharing - my girlfriend and I have always loved the idea. But, realistically, it can be very difficult to find someone who is equally interested in both of you. Usually, immediately or over time, two particular people in the triad take a shine to each other. Of course, there are many successful triads too - but it helps to know exactly what it is you are looking for.
Is there a reason you wouldn't want to date people separately? Jealousy issues in a triad can be just as rife as in a V, or other setup. What happens if you have wild sex all the time with the girl and your fiance realises he's getting left out? Or vice-versa? What if seeing your fiance snuggle with her on the sofa upsets you, rather than turns you on?
Have you guys talked and negotiated some basic poly guidelines? Things that are ok and things that aren't?
I have used Plenty of Fish and OKCupid to meet women. OKCupid is particularly poly-friendly. Lesbotronic is an interesting website that I just discovered yesterday - there is an option for polyamorous and bisexual/with a man/looking for a Unicorn. You submit your details and they do the matchmaking for you.
Me: (30f) open poly
GF: (40f) My long-term, long-distance partner
Hubby (37m): GF's husband
Garcon (26m): GF's submissive/third partner
“Peace comes from within. Do not seek it without." ~ Buddha