While it may naturally happen, it is far more difficult and unlikely to find a woman who will be equally into both of you. If the opportunity appears then great, but don't expect it.
Lots of poly people utilize OKCupid, and when setting up their profiles make note of the fact that they are poly. You can do searches for key words, like polyamory, polyamorous. And be certain to put it in your own profile so others can find you.
Depending on where you live, some of the bigger cities may have meets where you can go and socialize with other like minded individuals.