Gary,
Without looking at the parts list for your specific coach, my guess is the air dryer is a spin on Wabco cartridge. I have gotten those replacement cartridges from Fleet Pride and Truck Pro. They are common and readily available.
When I have dealt with Prevost Parts (I am sure Rob spoke to Charlie who is my preferred contact also) I ask for the trade number, not the Prevost number so I can shop the part. On a lot of parts I cannot beat Prevost prices or availability, but sometimes, such as when they are out of stock or their price is too high having the trade number enables me to shop the part locally. In the air section of the shop manual (available on line) you may actually find the air dryer manufacturer's model number and cartridge number.