I was surprised at how hard it was to find all the best carsets on my return to GP3 last year. It's taken me a few months but I now have good versions of all carsets from 1975 to 2010 (1992 was one of my few 'missing' carsets).
It would be good if they were all in one place (like the Downloads section of this site!) but you have to search all over the web or ask people to send them to you. I guess it comes down to getting author's permissions but some are so old now I can't really see anyone having problems with people uploading them. Why create them if you don't want people to use them?