I guess I'm a one of those mythical unicorns. How do you meet me? You just do. There isn't a specific place or way. However, I do want to throw in a bit of personal experience. When I met my SO, I knew he had another GF. At first we talked about how "nice" it would be if we could form a triad. We had great fantasies about how we would all mesh together. Then we were hit by reality. Triads are not easy. Period. We were not able to make it work as a triad. Now we are in a Vee. If you are truly going to seek a traid and that is the only relationship dynamic that is acceptable then be prepared for it taking a long time. Also think about what you are going to do when the triad doesn't work.
I don't think we are being hostile on here, more like realistic.