Email, email, email!
I landed a collaboration with a VERY talented vocalist (nameless for privacy) by simply emailing him my track and presenting the demo. Make sure the track you have is "pitch ready" before you go looking for a vocalist, because it will be much harder to line up a collaboration without a great track to pitch them. But if you think you're ready, find some songs with a vocal style you're looking for, and just look up their contact info and present the collab.
Feel free to ask with other questions, and good luck!
