If you are not very strict about the full time Open Source, you could try to find an employer that is also engaged in developing Open Source tools. Quite a few companies put a full or part-time developer aside to code on an OSS project they need to advance. Yet other companies allow employees to put forward projects that may be of interest to the company and then spend a week or two working on them.
So, if you'd already be happy with working here and there on something that is Open Source you'll have plenty more options. I guess the key to well being at work is to see some good purpose in one's work - and just participating in OSS software here and there at work may already give you just that. Also, when you company is actually not "evil" and the products they develop really help many people, that's quite rewarding.
If you really want to do just 100% open source, then look through the different relevant forums. Also, you could call for a list of companies currently hiring OSS developers (though for more opportunities you may have to be willing to move somewhere). A brainstorming would probably start with all kinds of bigger Linux distributions and other bigger OSS projects. A short check reveiled openings at
I couldn't find any openings at
And there are very many other companies that you'll have to check yourself or hope for tips from other's responses (looking through the openings, there are some OSS developer positions here and there). Among those are IBM, HP, Canonical, Qt Software, Sun, Mozilla, KDAB(.com), Oracle and many, many more. Also, the public administrations that are about to switch to OSS based solutions and colleges may be an idea to check. IMHO the OSS jobs offered demand a very high level of skills though. Good luck.