Just had a chance to have a look, after yesterdays hill climb, but I'm afraid I haven't found any official TT tshirts for 2023 anywhere.
The iomtt shop is the website Duke took over/bought from the organizers a while ago (now iomttraces.com) & they have some for £25, but I don't think they're "official" as they don't mention TT anywhere! (usually a sign!)
(after the trouble Dave at The Old Market went through a few years ago with his charity fund raising t shirts folk have to be careful these days!)