As this topic seems to come up fairly regularly and having several hardware synths, which I've either had to fish the net for defs or create my own, and in view of the fact that some online resources which older posts refer to, have now gone like the Cakewalk Net site, I thought I'd summarise some of the current ones which are left and also create a post with a title which should pop-up straightaway when searching the forum, which I hope comes across as helpful to people.
 
Firstly I'd suggest looking at this KB Article because it has links to a couple of INS file archives: http://www.cakewalk.com/Support/Knowledge-Base/2007013272/Instrument-Definitions#Locate
 
Secondly you could look here because there's another INS file archive here: http://www.lilchips.com/cakewalk.htm
 
Thirdly there's a free program which will generate INS files for a number of synths here: https://www.fumph.com/insmaker/
 
Fourthly there's the very comprehensive Cakewalk INS Tutorial here: https://www.cakewalk.com/Documentation?product=SONAR&language=3&help=Instrument_Defs.16.html
 
Fifthly there are some great examples and further instructions on how to make your own defs here: http://tweakheadz.com/how-to-build-a-cakewalk-ins-file/
 
And last but NOT least, there's Craig Anderton's excellent article at SOS here: https://www.soundonsound.com/techniques/sonar-using-instrument-definition-files
 
Hope that helps.  :-)
 
Anyone like to add other good resources or INS archives?

Thanks for this thread - I recently came across some difficulties searching as well. I just finished writing an .ins file for the Roland JV-1080 with Orchestral expansion I and II cards. I found that some of resources I came across had some flaws and I had to re write a few. Glad you're helping in keeping current resources up to date and I hope to contribute my little bit as well.