I spoke to Katie at Greenlight today about the GM production for these cars. They made 66 GM's. Six will be used for licensing rights and their archives. Sixty were sent to the distributor to do what he pleases. Therefore, there will be 60 Green Machines in circulation. The distributor is likely to keep some as well, so I would venture to say that there will be less than 60 available.
Guess no one wants to tell you anything Dave.
The website store lists an older Ertl Mustang for sale. Maybe you could contact them. Otherwise only ones I have seen for sale are from that Ad-Ventures seller on e-bay. He has a set of three colors up for bid.