Any shop that deals in dragboats should have them or point you to a shop that does. Off the top of my head, I'd check with Jim Brock at Cyclone Speed and Marine(in Pacoima, I think), Dan Aaseth at Canyon Marine in Newhall(ad at Dragboats.com, $85/each), maybe PM Cyclone or CS19 on HotBoat for leads since they seem to be at the front of the young dragracers these days, Prime Marine(somewhere in SoCal) does a lot of v-drive work and should be able to help you. I think Nucking Futz on HotBoat makes them, also.
Where ever you get the shells, they will still need to be upholstered. Try to use someone who has done them before as they will likely have a pattern, making the job easier, better looking and possibly less expensive.