It's certainly a DIY project.

www.infiltec.com

There's probably many more suppliers, but that's a place to start.
It's usual and customary to weasel some cash from the seller if you
still have an inspection contingency. My place had the same problem.
The seller just wanted the problem to go away so he offered $850 back
at closing, which I accepted. $250 will go towards the parts for the
fan/ducts and the other $600 will buy a lot of beer.



-rev