Question: There is a lot of track remixes online.
Most of producers use separated vocals from songs.
It is almost impossible that they could cut only vocal out of total track in such good quality.
What is more, anywhere in the network is not available finished dry vocal version.

So how is it possible that they are in possession of such good vocal version? 
Assume also that they have not purchased it from the artist.

Please tell me HOW?? O\_O

Thx

Answer: 
The generic term for split / separated tracks is "stems". If you google "vocal stems" you'll find free ones for random songs, not hits but usable. Vocal stems for popular songs are usually available (at a price) from the label or artist.
They purchased it from the artist, or possibly the record company. There is no other clean or effective option. Sound, and particularly voices are complex and once you mix sounds together, they can't be separated cleanly. It's kind of like if you mix water from one cup with water in another cup, you can't get the water back in the cups they came from.

If you had the exact copy of everything but the vocals, you could do a subtraction (invert phase and mix together), but if you don't have either the instruments or the vocal track, you can't separate them cleanly. Also, if anything was altered with an instrumental track, it wouldn't work right either, though it might still get a passable result.