Lesley,
Sorry to hear your sad news.
This site has a 'for sale' section.
You can also find 'for sale' in the Marlin Owners web site
http://www.marlinownersclub.com/
This site also sells interesting cars
http://www.carandclassic.com/
Failing that there is always ebay, though I'd recommend sticking a reserve on it.
I can't guess at the value, perhaps one of the 5exi builders can give you some guidance.
Chris