I'm working on a classic Funk composition wanting to add some vocal samples. I've searched hard, there seems to appear a lack of them available.

If vocals are the main instrument to popular songs why is this the case, I can only assume that it's because musicians prefer to add their own vox, or technology is not good enough to incorporate them into your own compositions.

Does anyone know where I might find any quality vocal samples?

I take it you know of www.acappellas4u.com ?
What about some real searching , put this in google search bar  " ?intitle:index.of wav vocals "   .... without quotes.
Swap the wav bit for mp3 and see what happens.

Still no joy....
I'll have to get the mic out I think or find a good singer locally