Welcome to the site! I think you're like many couples out there who, especially when first exploring poly, consider themselves looking only for a third individual (whether man or woman, usually woman it seems). This woman is the unicorn (a bi chick who wants you both). If you check out over this site, there are success stories, so it is possible.
However, seeking advice for meeting that person... it's very difficult to do so and can be disheartening when throwing yourself out. There aren't really dating sites geared toward what we search for (we are also a poly couple who has looked for a third in the past). One of the best ways might be to be active and open to meeting people, and forming friendships. When you're comfortable, let them know what you're looking for, or put feelers out and see if they're looking for the same.
Many people don't realize they'd be ok with the idea, they don't actively pursue it. This is why I'm not saying, go up to people and say "Hey, we'd like to play the Eiffel Tower with you, you in?"
in any case, good luck! Feel free to share your experience and questions here.
Him: the active-posting part.
Her: the brains of the operation
We're a young 20's couple seeking a third female to complete our triad.