I got them at a local HD truck parts supply house. They stock all of the major HD brands, rebuild a lot of parts etc.
A lot of the trucking companies use them. It is:
http://www.coltontrucksupply.com/
Apparently, they are NOW called Fleet Services...but it is the same place.
http://www.fleetservicesinc.com/
My post re: the filters and air dryer are under "C8.3 Service" under "class A".