Where to find loops, acapellas, and instrumentals? Here are as few ideas!
One thing I’ve had success with is emailing the manager directly. Just say you are interested in making a remix, that what you make wouldn’t be monetized, and ask if you can have the vocal stems. It’s worked a few times when I couldn’t find an acapella anywhere else, it’s actually where I got them for my latest remix. Manager sent over the original WAV files of the vocals, drums, piano, and fx.
Voclr.it is pretty neat. And for official remix competitions you have wavo and indaba.