You are right amobrasil, level headed people that don't need too much approval are rare. Which is why I thought it a shame after having talked to our unicorn that she was the typical "well whatever works for you guys". She wasn't a bisexual when I met her either.
So possibly you could broaden your search to non bisexuals. You basically just need to meet a lot of girls and find ones that are good, and go from there. I wouldn't go out looking for poly or bi people as my main criteria because they often don't fit the mould anyhow and are a rare species. Many girls are interested in trying things with another woman but wouldn't list themselves as bisexual, or even think of themselves as bisexual.
If you approach it logically you'll have a greater chance of finding someone I feel. Also I'd look at those 3 "encounters" you've had as growing experiences for you and your gf. You may think they have been a waste of time but in reality, it's honing your skills.