I'm on a swingers' forum, and there are quite a few people who identify as "in an open relationship" vs. "swingers" there. They are the purely sexual kind of open, so they wouldn't fit in here, really, but they are very well accepted in the swinging community (not that they wouldn't be accepted here, they just probably wouldn't relate as well).
Around here, it seems like most of the people who attend the open relationships events are the same ones that attend the poly ones and some of the same ones that attend swingers ones. There aren't many people (in my experience) who identify as open that can't fit into either a swingers group or a poly group and find people who understand and agree with their viewpoints.
Is there a particular brand of "open" that you're looking for and not finding represented?