My second-hand experience is that paid section leaders belong to a very self-protective, tight-knit group.

If there are any professional (or semi-professional) vocal ensembles in the area, check with them. These groups often have the best paid singers, and the members are usually the ones that hire out and also know who they can get as subs when they can't keep an obligation themselves.

Also, check with local AGO members to find out what other churches in the area employ paid section leaders. They can get you on the "inside track".

I would suggest it's "whom you know". Being in a metropolis and music school (well, department...), our profs do send out those e-mails, but we mostly ignore them. However, with the prospects of a job that may hire section leaders, I have approached other students and asked if they'd possibly be interested. Usually the response is good. If you or someone in your choir knows people in the universities, get the word out. We want to know we'll enjoy it too - be open about what you do. Very few students (they do exist) want to go sing a Haas ditty, but many will jump at the opportunity to do polyphony and anthems. Also consider the lesser schools. Having been at a community college, I can tell you those students will do anything for money, and they're usually very talented. Contact anywhere that has a music degree.

I second David's advice on professional and semi-prof groups. Two more thoughts-

1)You may want to contact music professors (including community college instructors) directly, and I'd go straight to the ones who have any kind of early music ensemble.
2) If there is an early music society in your city (likely with 3 mil population) I would contact those folks, and even see about getting an announcement in whatever they send to members. (Having your church in such a bulletin would be good for evangelization, too!)

Instrumentalists can work ok, especially as regards sight-reading, but beware of vocal quality and language issues, etc. A skilled singer will almost always serve you much better in terms of beautiful tone and sensitive phrasing. Your choir will be absorbing the way the section leaders sing overall, not just how they learn notes.

Congrats on being able to do this, and make sure (if you haven't already) your pastor knows that such an investment could take some time, and is worth being choosy.

When you interview them, test them rigourously in reading and sight-singing. There are many singers, be they professional, voice majors, or what, who cannot read well and often require as much tutoring as less credentialed singers. Singers are, notoriously, among the worst readers. And, of couse, you want to determine that they have good 'choral' voices and understand that they are not engaged to be soloists.

In addition to your section leaders' vocal ability (don't forget having voices that blend), you need to pay close attention on their suitability to this particular singing environment. They're not just there to be your reliable Sunday voices. You want them to support and encourage your volunteer singers - modeling good tone and diction, leading mini-sectionals, etc. A quartet of divas (male and female) can breed a lot of unhappiness.

Diocese of Oakland CA ...

http://www.oakdiocese.org/

(left column)
Online Publications & Forms
Chancery/Parish Personnel Policies
Appendix E