Firestone Ride-Rite
LOVE THEM, WOULDN'T BE WITHOUT THEM.
Bought mine 2 years ago online but I'd have to dig out the bill in order to tell you where. If memory serves me I bought them at Suspension Connection, I think.
If you are the least bit mechanically inclined you can install them yourself, I did.
Here's a couple links to online sales...
http://www.suspensionconnection.com/...V2gq8S#ITEMIMG
http://www.truckspring.com/air-spring-kits_256.html
BTW-As I have a compressor in the Monty I didn't bother with the optional compressor.