as soundtracks go, you want something that compliments the gameplay first and foremost, from there its anything goes really.
VG remixes are always a good bet, since the tune is copyrighted, the artists have to either give out the tune free of charge or buy the rights to it. Almost all are simply put on the internet for free.As far as SF remixes go, most of what you'll run into are Corneria remixes, some good some bad but almost all Corneria with only a few exeptions.
Here are some of what I've been able to dig up on the web:
From:
http://www.ep-melody.com/en/html/music_other.html
This is a Japanese artist and doesn't use the usual download format, but his remixes are great. when you click on the link for his music pieces you will be taken to a blank page where the music will be playing. to download it you have to open the blank page w/ music, click File then Save Page as... (mp3 format should be default) and save it as you would any other file
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Zoness
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Corneria (This is a great mix, but I really request you don't use it since I'm planning on using this piece in Perfect Chaos III)
From:
http://strike911.sanctuslegacy.com/music.html
This guy is more about the trancy tunes, but there is some good stuff in here. He uses the usual download format.
http://www.newgrounds.com/audio/
The newgrounds audio portal is one way to go, it takes a lot of digging but there are diamonds in the rough if you take the time to sift through it all.
http://www.ocremix.org/
This is a very big site with thousands of remixes of almost every VG and PC game out there
http://www.soundclick.com/genres/charts.cfm?genre=Electronica&genreid=17&showonly=0&orderCharts=1&advstate=0&advcountry=0&advvip=0&advfeatured=0&advsigned=2&subgenreid=83¤tPage=1
This is another good place if you like to dig, but there is some good stuff in here also
If you can find an individual artist's webpage, that'd be a good place to find some tunes