Ok so just went and checked --- the penthouse seal appears to use common automotive push-in plastic body/trim fasteners.
You can actually find those in the "help/fix-it" racks of frequently-needed repair parts at many auto parts stores (although not in the significant number you'd need to do an entire PH roof seal....)
^^ those ones that scalf77 linked to look identical to the the ones on my 23-year-old penthouse seal
....could be a good price/deal for the sheer number of them you'll need to do the job.
As a technical point:
IMHO, they *aren't* rivets, they're push-in automotive body/trim fasteners/retainers. A true rivet has a sliding internal pin (or screw) that draws the ends closer to each other and thus expands, to create a secure attachment. But I guess anyone can call anything a rivet these days. You will probably have better luck searching for a match to the SMB penthouse seal attachment fasteners if you search for "automotive trim fasteners" instead of trying to search for "plastic rivets"......
In any event --- if anyone is interested.....
Here's a some pics of the kickass (and of course inexpensive) Harbor Freight plastic riveter set
, along with some pics of how the plastic rivets expand. These work great on "blind" fastening situations where you don't have access to the backside of two pieces you're attaching to each other. A ton of modern automotive trim requires this fastener approach to properly affix components to each other. (Inner plastic fenderwell to edges of front plastic fascia on many cars, for example....)
It would definitely* work on the penthouse seal as well, just another approach.
*most likely? Need to double-check the OD of the penthouse clips and the hole they affix through.