Did you call Quadrafire to get a price on the part? That might save a lot of hassle if it's reasonable. Otherwise, on the firebox side I found this material to pack around the manifold pipes which sounds like it might work:
http://cgi.ebay.com/Nelson-Firestop...863?pt=LH_DefaultDomain_0&hash=item255ef00b97
http://cgi.ebay.com/Nelson-Firestop...863?pt=LH_DefaultDomain_0&hash=item255ef00b97