Well now, that is interesting. I work part-time at Cimarron and haven't heard anything about grip size changing. I emailed Chris Ballard at Cimarron and he said he hadn't heard that but "it wouldn't surprise him". I probably won't be back there for a month so can't measure them. But I did measure my own Thunderer. It is over 10 years old, and I have several sets of grips for it. Wood grips on it are 3.18". I have a set of unfitted Buffalo Bros. grips that also measure 3.18". Another pair of unfitted wood grips are 3.16". If I was to guess, it might be that SS guns have a slightly longer gripframe thus the longer grip, but that is a guess without having access to other new guns to measure. Now back to your original question, Vintage Gun Grips in Florida carries some Thunderer grips:
http://vintagegungrips.net/ao-u13.html - these people make these hard rubber/plastic grips for Cimarron as well, and I have a pair. The Cimarron ones have the Cimarron rider in the oval at the top, but the ones on their website are just blank, which looks kind of odd. Their site says they are available in ivory color. Now here's the bad news on them. My digital caliper decided to stop working (I just changed the battery recently so maybe bad connection?) but comparing mine to the Buffalo Bros. grips, the Vintage Gun Grips grip is slightly shorter (maybe 3"). So probably too short for your gun. Good luck on your quest!