Welcome to the site.
As a gay poly guy, I do not know of any polyamory sites or services that are LGBT-specific. I really think starting with you local poly community and your local lgbt community---and trying to form a bridge between the two---is your best bet. It has worked for me so far, as I do outreach & awareness activities between the two, and have met my most recent significant other that way!
For meeting people online there's polymatchmaker.com as the main poly-only dating site and then OKcupid is a mainstream site that's considered the most poly-friendly.