Where would you look to find great tech writers to write user manuals for software? Are there specific sites, areas of the web, etc. that are best to look for them?
Aside from the usual places to find people to fill a position (résumé and job posting sites and networking), you may want to contact the Society for Technical Communication chapter in your area. Many chapters maintain a mailing list or have a specific person who helps communicate job opportunities to members. The Society at large also has a jobs site.
I'm a technical writer (who sometimes ends up programming, FWIW) and I've been quite involved with STC in the past. There are plenty of talented and professional STC members, so it's definitely a resource I'd consider.
It's a standard position in IT industry. Do whatever you usually do to find developers, or system administrators, or testers. Contact a recruting agency, post a vacancy on some Web site, etc.
Ask for writing samples and if you like the docs and they describe something similar to what your company does, it's your writer.