These are the main feeds that I keep in my RSS reader. I scan for iOS-related posts, even when I'm not actively seeking work. If the post looks decent, then I check out the posting company's website and it's usually obvious whether or not they're a good design firm. If I find a good one, I'll email just to connect for future potential work.

* Stack Overflow - http://careers.stackoverflow.com/

* GitHub - http://jobs.github.com/

* Get Apps Done - http://www.getappsdone.com/

* Authentic Jobs - http://www.authenticjobs.com/

* CrunchBoard - http://www.crunchboard.com/jobs/

* Mashable Jobs - http://mashable.jobamatic.com/a/jbb/find-jobs

It might take some time, but just a couple firms can keep you busy for years!