If you find that there isn't a local advocacy group, then you're the local advocacy group. Who's going to say you're not?
Years ago I heard a talk by Dave Meslin, a well-known Toronto organizer and one of the founders of the Toronto Cyclists Union. One of his points that stuck with me was that for many of the things he worked on, he just made up an official-sounding name for it, so that his letters to officials sounded more authoritative. Basically, if you write as anthoniix then you're easy to pass off as just some local kook, but if you write as something like "<your city> Bike Lane Committee" then you sound official and organized, and you're more likely to get a response from the bureaucracy and the media. Plus you'll probably find more people interested in getting involved if you have a name for it.