HI,
Informally they can be found at
Ecuadorian soccer matches.
About a year ago I was able to get one made by request at Salamera on Clark, which has since closed. I am glad to have the opportunity to taste it, though it is not my favorite.
There is a Monica Eng article in the Chicago Tribune archives on cuy. Her husband is famously quoted as finding the taste similar to "Fish bacon." The article is from some years ago, dated enough the places she visited may or may not exist any longer.
You may learn who makes this by making phone calls to Ecuadorian restaurants, though who knows how they may respond. In person you may have better luck especially if it is an off-menu item.
Regards,