There are no reproductions. If you find a used one, it still may need attention. Have you considered having yours refurbished? It is a common practice in the field of general aviation. In the past we have recommended this course for this 71-72 only style tank.
A member here mentioned C and C Auto Ranch has up to 18 seventies Buick Rivieras. Not likely cheap, but that might be the fastest way to get one.
The real question is the condition of the core tank he will be paying a lot of money to buy and ship. Here is another option. Split the tank and redo it. http://www.secondchancegarage.com/public4/gas-tank-repair-1.cfm
