Finding Official Instrumentals
If there is an official instrumental available for your track it will most likely have come from one of the following sources:
1. The studio instrumental had an official commercial release. Dance, Pop and HipHop songs often may have official commercially released instrumentals. In particular a lot of 80s songs had officially released instrumentals in the form of B-Sides. Rock and indie songs tend not to have any official commercially released instrumentals.
2. The official instrumental version of a track was available on a promotional release (also known simply as promos). Often instrumental versions of albums and singles are distributed for use as background music on radio and television. This is a good way to find many rock and pop instrumentals. In particular the Strictly Background series of compilations features many modern songs across various genres. As well as for television and radio use promotional releases are also often distributed to DJs too.
3. The song was used on a game such as RockBand or Guitar Hero. If the song has been used on Guitar Hero or Rock Band then it is likely someone will have obtained the instrumental and acapella tracks from the stem files on the game. Stems are simply the separate parts of a song - for instance a complete song on the game may have a Guitar stem, Bass stem, Drum stem and Vocal stem - and when they are played together they form the complete song. Note that this only works for tracks where the songs used in the game feature separate stems, as opposed to fan made guitar hero 'tracks' or similar which simply use the original audio track.
4. The Stems of the track were made available as part of a remix competition. You may have heard bands and artists running remix competitions where fans get a chance to remix one of the artist's tracks, and usually the best remixes submitted will win a prize or even appear on an official release. For people to be able to remix the track the stems of the track have to be released to the public, and from these the instrumental track can be recreated. As well as remix competitions sometimes artists will freely release the stems of a track simply out of the kindness-of-their-hearts too!
5. The song was available in the U-MYX format. Some songs around the mid noughties were released with remix software which allowed you to remix the track - thus also allowing the instrumental and acapella version of the track to be obtained.
6. The artist or someone else with access to their music leaks the official instrumental.
Finding Official Acapellas
Described above are the main ways it is possible to obtain an official instrumental of a track. Most of these also apply to finding the acapella version of a track. The main differences are that acapellas are not distributed as promotional background music (but may be included on promotional releases intended for DJs) and commercially released acapellas are mostly confined to rap and hip hop tracks.