Once again, it is Cessna Pilots Association (CPA) to the rescue! CPA is a "type club" (and an excellent one, at that). It maintains an extensive web site with a very active members forum. I posted a query in the "Maintenance" section and in a couple of hours I had a response from a member in California that provided the part numbers I needed.
Y'all pay 'tenshun here: If you fly a Cessna you cannot make a better expenditure than the few bucks ($55, I think) for annual membership in CPA. Every year without fail, I learn things that save me far more than the membership fee. CPA is a Very Good Thing!
Anyway, now I know that I need p/n 0752016-7, officially an Expander Boot. Either of the previously referenced sources will sell me one for about $75. By the way, it is most fortunate that I only need the silicone rubber boot and not the entire assembly including the associated aluminum bits. The assembly would cost $1,318.50. (Ouch!)
Let me put in a few words of praise for the Cessna Aircraft Co. on the subject of parts. OK, they aren't cheap. But as a matter of policy, Cessna sees to it that I can get the parts I need to maintain my 32 year old airplane. Not every owner of an older airframe has that comfort level.
I shared the results of my research with Three Wing and they assure me that they are on track to get the boot in-house in time to support completion of the annual by Friday.
So far, so good...