Yeah, thats one of those questions that has been asked a zillion times. The search would come up with all the results you need, but here is the short answer:
Riff and Mark had some custom work done. I don't remember for certain, but this might have been the one Riff used (I'm too lazy to do a search myself)
http://www.updplastics.com/
Mark used Fantastic Plastics in Stuart, Fl (I think)
Other good places to look if you don't want custom stuff are:
Marine Connection Liquidators:
http://www.marconlq.biz/
Don's Salvage:
http://donssalvagecom.verizonsupersite.com/homepage1/
Flounder Pounder Marine
http://www.fpmarine.com/Plastics.htm
good luck...