Agree. I also would like to start freelancing, but all the ads that I see are for web stuff involving Ruby/Rails or Javascript frameworks. Nothing involving back end work.
I hope I'm not being nitpicky, but Rails is a back-end framework, and (most likely) some of the Javascript frameworks you're talking about are also back-end frameworks.
I don't know for sure (I started out as a C++ developer, but I'm a web dev now) - but I'd suspect there is a lot of work out there. A better way to test might be to go to a local developer meet up specific to your skills and ask people where they work. I'd be surprised if there wasn't plenty of work to be had.