I am a terrible singer, but have gotten better results than using samples by singing myself. I sing 3 versions of each note and pitch correct with melodyne(heavily, I might add). I dloaded this free MAutopitch app from Melda Productions...get it here:
https://www.meldaproduction.com/MFreeEffectsBundle I use the "Format" control to change the timber of each note so none of the notes sound like me. Some sound like women...the higher the pitch, the more I try to go up to the women sound. The lower the note the more I go lower with the format setting giving it a huskier man's voice.
You get better results doing it this way 'cause when you change from Ohh to oooh to ahh or whatever, it sounds much more natural, and you have the ability to use any syllable you'd like. Throw all the vocal takes on to their own bus, add a ton of compression, some reverb, and you can have any size choir singing any vowels. Much better than some sample library thingy that sounds out of place in your mix
Try It! It takes some time, especially the 1st time you try. You'll be much happier.
by the way...the best vocal samples come out of Spectronics' Omnisphere.