I would also add that you're going to have an easier time finding partners if you are open to relationships that fill your needs in a variety of forms. If you're going at this with a specific end structure in mind ("I want to find two girlfriends to be in a closed poly-fi triad with with me") you'll shut off a lot of possibilities for how your needs could be met (though I recognize that you said you're open to other relationships). Another approach would be "I want to find partners who are non-monogamous like me".
If you keep putting feelers out, you may find that there are non-monogamous women already in relationships who might be available. That may be a way to put in you touch with any communities there if there are any. Though you really do have a challenge there. Hispanic cultures generally have a very different view of open ethical non-monogamy and the one Hispanic person I know that has an open poly relationship is pretty much cut off from her culture because of it. Some cultures exact a much higher price for difference of relationships than others. (partially why most poly people are white middle class)
As a poly single person, I've found it can be frustrating but generally my reasons seem to be different than yours. I've never left room in any of my relationships for the other person to assume monogamy. I definitely agree with RP in that you *have* to be proactively open about what you're looking for from the get go and don't allow relationships to move forward on unspoken assumptions. If you cut the assumptions out from the get go, you won't have any trouble upsetting any mono-only girls because they will just know that you're not a good partner choice for them and you'll know that they're not a good partner choice for you.