Pofessional? Probably buy them.
Non-professional? Everywhere you want :')
If you want acapellas, look for motown, rnb, soul stuff, it's used alot in dnb, and for a reason. It just works really well.
For vocal samples, litterally everywhere. Games, movies, songs, interviews, you can record yourself,...
Really, sometimes I just rewatch a movie because it might have some great stuff in there to sample. (And most of the time I forget to listen for samples because I get involved in the story again)
Interviews are also really good. The interviewers also want clean sound, so most of the time there isn't much background noise, and you can look for interviews about certain subjects that you want (for example about deep space)
Don't worry about the 'professional'
Smack some reverb on there and nobody hears the small crackles or the low quality