Usually just a quick Google search or looking around online to see if I can find the acapella I'm looking for. There's a few subreddits on Reddit that are dedicated towards finding and sharing acapellas/stems, such as r/songstems. Other times artists will host remix contests and release the acapella for a particular song, so you could get them that way too.
Also, if you have an instrumental for a song, you can utilize phase inversion with the instrumental+original and sometimes get a clean acapella. It varies from time to time though, I tried doing that with some Madeon instrumentals and most of the time certain elements would remain.

This drives me nuts. I love gettings vocals in but actually finding them without recording them yourself is a massive pain.
You can try to make them yourself though the results are usually pretty bad. Other people do it but usually it's pop music that gets the treatment and once again, they usually sound rubbish. There are also some websites but once again, they tend to be rubbish.
I guess you'll have to search for old disco accapellas like everybody used to do.