Source for heater ducting?

[e rock]

Forgive the newb question (I did search)... My newly inherited 71 is in dire need of a few minor things before going into daily service. Most are easily remedied, but I haven't had a chance to fully survey the heater/defroster ducts. I was told that the ducts are all rotted out, and based on the defroster's performance, I believe it.

Are the ducts specific, or will the generic spiral hose-type stuff work fine? If they're specific, is there a preferred place to buy them?

Thanks,
Eric