Trying to find a list of all singers who've ever sung.

Good luck You just want singers and not groups?
Just some of the ones I can think of

Neil Diamond
Neil Sedaka
Bobby Hatfield (Righteous Brothers)
Johnny and Joe (sang Over the Mountain)
Elton John
Elvis Presley
Ray Charles
Janis Joplin
Bette Midler
Aron Neville
Buckwheat Zydeco
Chuck Berry
Fats Domino
Chubby Checkers
Big Joe Turner
Tina Turner
Dwight Yokum
Alan Jackson

Category:American male singers - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Category:American_male_singers)

Category:American female singers - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Category:American_female_singers)

Contact BMI or ASCAP

A lot of the people that have sung may have had no agents or representatives of any kind, but could still be considered to be artists. So, they might not be found by asking any company or by using a search on the Internet.

It would very much depend on what a person considers to be the definition of an "artist" as far as that which might be about entertainers.