insulation inside air handler

I think it's called ductboard. It's the insulation that lines the inside of the air handler. Well, mine is smelling rather musty from leaks that have happened in the past and I want to replace the foamboard/insulation/ductboard (whatever you call it) and re-line the inside of the air handler.
I can't find it at the box stores, although they do have the typical foam board that is used inside walls.
What can I do?

OK, so I'm a little more familiar with what I'm dealing with.......

What I'm referring to is indeed called duct board. And the location I'm talking about isn't exactly the air handler, but the plenum.

What I did today was removed the blower, took it apart, and thoroughly cleaned it. I vacuumed the inside walls of the air handler real good and wiped down the rest with a rag and a bleach/water solution.

After doing all that, I'm kinda thinking that the plenum may not be so bad after all. But I still don't know where to get duct board at. I guess I'll have to call a Mom and Pop Hardware store and get some referals.

When I had the blower out out, I looked up into the ductwork. The heater coils looked good, but the main duct that is on the top of the air handler (is that called the trunk?) was, as best I could see, pretty nasty.

At least for the time being it all smells much better!

Ductboard is available at the heating supply house.Its available in 1,1 1/2,and 2 inch thickness.Usually comes in a 10 foot x 4 foot sheet,and you will need to cut it to the desired dimensions.The store will probably have a machine that cuts and grooves the board for folding and if they are not too busy they might help you build your plenum.