http://www.jamtrax.com/ http://www.backingtracksforguitarists.co.uk/ http://www.users.globalnet.co.uk/~steveml/page2.htm
If you like The Shadows: http://members.aol.com/cutcheon/index.html
These are just a few, some are live and others professionally sequenced.
Why would you cinvert midi to mp3? Because today you can find VST sounds that are so real you can't tell it from the real thing.
Try Bass Broomstick and BFD drums for starters. There is no keyboard out there with these sounds, then again their files are Gb sized and should be good. Then when your midi files sounds are replaced with these VST sounds your midifile is a totally different product, believe me! That's the answer why I choose this. I am always stribing to get and sound better!

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Roy-Andrè