Depends on what product you need them for. For instance, a quick Google for "instrument definition files" brught up a link for Yamaha products (
http://www.jososoft.dk/yamaha/ins_files.htm). Using that same search term you could probably find others in the search results. Maybe even tack on the name of a brand or model for more targeted results. (ex: instrument definition files for roland keyboard)
And then there's this, if you want to get your hands dirty:
http://tweakheadz.com/how...d-a-cakewalk-ins-file/