Nah, we're not mocking you. We just find it amusing that so many people decide they want to try on poly for size and immediately think there are lots of hot, bi babes just waiting to take up with a couple. It happens so very often, which can be seen by the way in which it is referred--hunting unicorns.
Seriously, have you considered thoroughly exactly what you're wanting? A bi woman to get involved with the two of you--obstensibly strongly tied to each of you equally--and that she not be involved with anybody else?
First, to expect somebody to connect with two different people equally is unrealistic. Heck, to expect somebody to connect with two different people at the same time _at all_ is unrealistic; any given woman is unlikely to find each of you a strong match.
The two of you are unlikely to find any woman strongly appealing to both of you; likely one of you won't be as attracted as the other. Have you considered how unlikely it is to find somebody who finds both of you strongly appealing and both of you, in turn, find her strongly appealing? (Moving beyond initial attraction, of course, and considering long-term compatibility.)
Then, to expect that a poly woman not be attached to anybody else stretches things further. If she's poly, it means she's into multiple relationships. Yes, there are poly triads to be found in many places (though many more vees when it comes to tangles of three). Most of those also involve outside connections, too--most aren't closed relationships. Of those that are closed triads, most of those appear to have developed organically out of previous open tangles.
So, we never chide people about unicorn hunting out of malice. We're making light of a common show of naivete among those coming newly into the poly world. It's a healthier response for us when compared to ripping out hair out over yet another such appearance. (My hair is thinning without that.)
Further, we don't discourage such a hunt, either. If you can find exactly what you're wanting, we're happy for you! That means you've beaten the odds.
I'll also add that you're more likely to find a unicorn among women who aren't currently poly. Poly-curious mono women are more likely to find the idea of a small, closed group to be a safe way to experiment with poly. I suspect you already know somebody who's both bi-curious and poly-curious, though you're probably not aware of her possessing those traits.
So, welcome to the boards. I don't get to read as much as I'd like due to having to scan for spam posts much of the time I have available, so I've not been able to welcome everybody to the boards as I'd like.