There's no such thing as an "open source job."
There are jobs where open source is used as the platform for development and deployment. There are jobs where open source is used, but the company culture is one where "We take, but we never share." And there are jobs where open source is used and the company culture encourages community engagement with the people who provide the platform. I've worked at all of these.
I find that companies with a culture of community engagement get to market faster and survive longer. I worked at a company where I was allowed to send bugfixes, patches, and extensions to Linux drivers, the Python standard library, and the Apache mod_log plugin; it was bought by a bigger company, lawyers got involved, all this "sharing" had to stop-- and the company tanked two years later.
I can understand not wanting to work for a company that has closed its doors to community involvement. But I've worked for a consulting firm that used MS products and contributed what work that wasn't it's core intellectual property back to the community.
If you don't feel that you can be productive in an environment where upper management has decided to lock the doors on core development and contribution, and tells you that your duty is to "work with or around the bugs, frustrations, and so forth" in MS products (I've got your SharePoint Horror Stories right here buddy), and you want to leave... more power to you.
And ignore the whingers who say you should be "grateful you have a job at all." If your corporate master is gonna screw you, screw 'em back and take your experience and skills elsewhere.
My best experience with "open source employment" is to put the things I know on my resume (http://www.elfsternberg.com/resume/), then send the resume out to people who use the tools you know best. Put it on Monster, and update it every two weeks: just deleting it and reposting it will make the recruiters call you. I know: Python, Perl, Django, Rails, Ruby, MySQL, Postgres, S3, EC2, AWS, Git, Subversion, LAMP, and a ton of other things in the end-to-end stack of web development: I can go from having a box of parts and a Gentoo boot disk to a full-sized website with Responsive Design, Database backing both SQL and NoSQL, and Ajax and Socket.IO sexiness in a day.
Also, find the craigslist in your area. Get yourself an RSS reader. For me, the feeds I took from Craigslist were "Web/HTML/Info", "Internet Engineering", "Software/DBA/QA", and "Computer GIGS" (the last is for short-term contracts... I've made $1000 in one day with some of those). Scan them every morning, pull up the interesting ones in your browser, and send them a resume. Have several, tailored to different skillsets, along with cover letters. You might get one hit for every twenty you send out. Also, if you're in a decent-sized city, you might find it has a "startup community." Check their blog-- startups love open source, and they love good talent. They might even have a job feed-- Startuply in Seattle does.
Good luck finding a new job.