Many kits use the tank from their donor car and here’s a great source for OEM fuel tanks: http://www.quantaproducts.com
If I don’t swap in a racing style fuel cell when/if I get the Pink MG back on the road, I may go here for a replacement Chevette tank.
I bought a tank from Quanta for my ’79 Chevy Caprice Sport Coupe. Good stuff. I’ve never had good luck flushing gas tanks. Besides, it’s no cheaper than buying a new tank in most cases.