However, if a brand manufacturer has an old product delisted, then physicians can no longer prescribe generic equivalents. That's become more and more common.
However, there is a workaround: compounding pharmacies. To my knowledge, accredited compounding pharmacies can make, on a customer-by-customer basis, any product that the FDA has ever approved. And many of them are mail-order pharmacies. In my experience, prices are comparable to mass-market generics, or lower.