I also recommend reading, So Someone Called You A Unicorn Hunter.
Generally it's a bad idea to go out there as a married couple expecting to find a HBB to "share." Sometimes you can find a gal interested in a bit of 3way SEX, sure. You might end up in bed with someone and have a little fun a time or 3. But having long term caring relationships (which is poly, not just fucking around) is whole other kettle of fish.
Most polyamorous couples date separately and don't have group sex. But often married couples will think, "Oh, we want to do this together!" Well, a new female partner isn't a sex toy to be shared. She's a full person with feelings and needs of her own. She might prefer one or the other of a couple after the NRE wears off. One of you might end up not getting along with her, while the other one is fine. Then what do you do? Do you have a veto where the Unicorn must be dumped if she doesn't fit your parameters of a shared experience where all sex with her, and dates with her, are a threesome deal? Or, is it OK to have one on one sex with her, and one on one dates?
Is it OK if she is poly and has another partner already? Is it OK if she has kids? Is it OK if she doesn't want to live with you? Is it OK for you both to move with her if she gets a job in another state? If one of you gets a job in another state, and she doesn't want to move, what then?
All that said, you can go on OK Cupid or Fetlife and try. But please spend time reading first so you understand the pitfalls of unicorn hunting, to lessen the potential disasters.
Opening Up (book)
More Than Two (book and website)