I've had some success meeting poly people on OKC
. If you make sure to mention it in your profile, OKC will match that up with references in other people's profiles. You can also answer lots of the "poly type" questions to improve your match, making sure to list those answers as very important (things like, would it bother you if your partner fell in love with another person).
As for meeting people in real life, I think the best approach is to ask if they've ever heard of polyamory. If they have, ask them what they know about it, if they haven't, explain what it means to you.
Also, when you said "open relationships" and then they "cheat" could you be more specific? What boundaries did you set out alongside "open"? I've found that the term "open relationship" has a lot of interpretations, so if things were never clarified, I can see how there might be some misunderstandings. I personally try to stay away from that term for just that reason.