The Hot Bi Babe who is looking to get involved with a couple is called a Unicorn for this reason--it's a mythical creature! There generally aren't women seeking a couple to get involved with, though some are certainly open to it, should it happen.
The problem is that it's unlikely to find somebody who's going to connect with you both equally. The problem is compounded if you're expecting to find somebody to connect with the two of you and not be involved with anybody else. If you do find somebody interested in the two of you, she's likely to share a stronger attachment with one of you. That can work, as long as you don't mind the uneven attachment.
If you really want to hunt a unicorn, I'd suggest looking for a woman who's never been in a poly relationship before. I suspect it's more likely to find a woman open to a poly arrangement than one experienced with it. (Actually, we know a couple who has found girlfriends in that fashion with some regular success, though the women are usually involved with somebody else, and the involvement with the couple is a first foray into poly.)