I have a motorhome conversion on a 2006 Freightliner M2-106. The cab air ride is fully operational and, as you mentioned, there's an accordian seal between the cab and the coach.
The type of seal that you'll need depends on how the cutouts in the cab and coach are configured. A Google search revealed some possibilities:
All-Rite
Elasto Proxy
You might also get some info from the manufacturer of my conversion,
Transport Designs.
Hope this gets you started.