A lot of 'a capellas' you find around the net are actually just stems from the original recording that have been kept separate for remix purposes or otherwise. You can find a lot of these vocal tracks simply by finding open remix contests, or vocal sources people have saved from previous contests.
Find a Remix Contest | Remix Comps
ACAPELLAS4U - 7 years on and still satisfying your lust for free acapella downloads • Portal
Regarding using equalization to isolate vocals, that's a lot more difficult to achieve with any kind of acceptable quality, as in many cases other instrumentation share the same frequency range that vocals occupy in the mix, which makes them impossible to really separate from the vocals.