Separating vocals from beat to get an acapella?
There are “acapella” samples in youtube which, what the uploader did was he really soften the background beat and made the vocals louder. I’ve encountered a few of these samples in youtube.
Though not 100% perfect, but still good, I’m wondering how do I achieve this?
Dynamic filtering would be my first guess. depends a lot on the source material how decent it will be.
Keep in mind that some are indeed the acapella, but the singers tend to love loud headphones so you can hear that in the recording.
having the instrumental & original track stacked with phase inversion to push the acapella out. you’ll still get some bleed thru but it would be a pretty decent accapella.
you need to get hold of exact copies of the instrumental and the original track for this to work (literally an instrumental version where the mastering engineer has muted the vocals on the mixing desk whilst exporting the 2 versions).
you then need to line the wav files up perfectly.
it’s not easy - I’ve tried a few times 
You can try converting to mono and/or flipping the polarity of one side of the stereo signal. Depending on how it was mastered and how things are panned, sometimes that kinda works.
Otherwise, try to buy them…or re-create and re-record what you need.
The short version is that you can’t do that.
Where can you buy original acapellas? You say “try” and that is something I also noticed. Finding any original acapellas is hard to do. With the digital day in age, you’d think selling acapellas could be another income source.
scamo
They’re in the dj tools section on beatport if beatport has them. Other sites have similar names.
But, yeah…that’s one of the things tht got lost with the switch to digital…a lot of hip hop records had the instrumental and the a’capella on the single.
Also, remix competitions often have the vocal stem, which is sometimes usable depending on the licensing.
Other than that, googling sometimes can find some.
I remember trying to do this from a vinyl rip… using a similar part from the same track without the vox… having warped the two parts.
Now that was challenging! (and totally unfruitful )