You can sometimes find vintage t shirt gems such as “F The FCC” and “Howard Stern for Governor" available online. Show everyone how much of a hard core fan you are and scare strangers.
While Howard has been very guarded about protecting his brand and not releasing a slew of T shirts, fans have been producing T shirts about the show for decades.
There are new fan generated t shirts coming out all the time. Some of them are pretty good, but you do have to wade through a number of the usual Sirius fists to find them.