If you can find it, get a copy of Mashtactic. It may accomplish what you are trying to do in getting rid of the bass frequencies etc. Good luck :) http://www.youtube.com/watch?v=0lDAY0va4VE
Also, I've just downloaded the trial version of ADX-Trax and it looks absolutely great. Granted, I'm testing it with a semi-separated central channel audio, but the first results are very good. Their demo video tutorials show how cool this software is (and they use it to demonstrate how to separate Adele's "Rolling In The Deep" vocals from the instrumental). Is this what people are using to extract vocals these days? Could very well be. :)
i would like to hear that Glamour dude chime in here, some of his diy pellas are as good as you can get, he did a great job with the pharrel happy one to..
If you have the original track and the original instrumental, load them both in Audacity and invert the instrumental. It will not sound perfect. There will be some bleed, but it can be made good enough for remixing, depending on what you do with it and how meticulous you are. https://www.youtube.com/watch?v=_p5QC-4jGWA
There really isn't a magic method to making acapellas. DIY acapellas are a process which differs from song to song. But, you could always try contacting the artist of the song you want an acapella from, twitter, facebook, e-mail, etc. Be friendly, introduce yourself, say you're a big fan and would like to remix a song of theirs, whether it's for your own creative advancement or to release. Obviously the response time will probably depend on the size of artist's following, but hey, you could even maybe get stems out of it if the artist is okay with it. Also, a big thing is proving that you're trustworthy / legit. A lot of bands or artists might consider stems or acapellas kind of "top-secret"-ish material, so they probably won't be totally comfortable with giving it out to someone who could likely just upload it to a popular site for the whole population of the internet to gnab. So with that, include links to your soundcloud / bandcamp / facebook / twitter / myspace / aol / club penguin account to show them that you are capable of making music and are not some internet randie, this will give them a better understanding of who you are and it'll make them more comfortable with giving you things. If that doesn't work, well... you could always try bootlegging? :D
Ripping from original tapes is a little easier. Especially the old 8-Tracks.
There is a machine made for those tapes that plays 3.0 'Surround' with the Vocals only in the middle.
Quite fun, but it's not around anymore, cheapest I saw ever was around $5,000
Some cassette's and CD's have official released instrumentals on them, which leave out 3 specific frequencies, which you can find out online and remove them yourself once you make an inverted DIY.