It would seem that turning up a good singer would be relatively hard, but I have not found that to be the case. Over the years, I have encountered a number of excellent vocalists through groups that I have worked with. However, I have had far better luck with running an audition call in the local "arts" paper - the alternative weeklies, as they were once known.
I have a standing ad that I can place with a phone call to my "advertising rep", a pleasant young man with a lot of piercings. It reads:
"Wanted:Musicians & Vocalists. Perform with a Vegas style show band that works throughout the Gulf Coast area. (Saxophonists, trombone players, trumpet players, rhythm players, vocalists) needed. All musicians must be able to read music! Call 832 630 8546 for audition information, dates and times."
(The text in the parentheses is altered as needed, depending on what I am looking for at the time.)
If you run the ad in a large city like Houston for a month, you will net in such a crop of vocalists (and musicians) as you have never seen. For, while the ability to sing is widespread, the opportunities to sing are few and far between.
Once you verbally screen out the "Well, I don't really read music, but I pick it up quick..." sidemen from the mix, you invite the remainder (vocalists and musicians) around to a rehearsal and see what they can do. I give them a list of standard tunes and keys, ask them what they have done in the past, and then run the tune with one of our current vocalists to give them the road map.
(We also lay in extra coffee and donuts. The way that some of them eat, you would think that they were there for the food.)
After each takes a pass at about three or four tunes, you can almost always pick out who will work, and who will not. It is a bit tiresome to hear Fever so many times in one day, but the end result is worth the effort.
With the horn players, it's even easier as you can tell who can read well and who can't within the space of a single chart.
At the end of the process, I offer comments concerning the performances. Some take this well, others don't. Usually, the sidemen and vocalists know how they've done before I say a word. I always try to be positive as possible, but honesty here is always the best policy.
(Once, a woman who was just not up to our level was politely told that she wouldn't fit. She sent me an email later maintaining that she was not a lush, and that I shouldn't believe what I had heard. (I had heard nothing - she just couldn't sing a lick.))
At that point, I explain how we operate, how the shares are drawn up, and what's expected of them. The ones who accept are brought in - sometimes to replace someone who's moving on, sometimes to start anew.
And, occasionally you get lucky. I've found a vocalist who is an excellent guitar player in all styles - he's been with us for years. Ditto a trombone player. And, some of my "mainline" vocalists are quick to handle auxiliary percussion as well - my "Sinatra guy" plays Latin percussion as well; timbales and congas.
I'd be interested in hearing how others fill their vocalist slots. No ribald humor, please...