Hi smithl5,
There is more information in post #2 here about buying a waterbed insert that should be helpful.
The better options or possibilities I’m aware of in and around the Cincinnati, OH area (subject to making sure that any mattress you are considering meets your criteria and the quality/value guidelines here) are listed in post #212 here.
The forum list for the Chattanooga, TN area is in post #2 here.
You may need to make a few calls to see which of them can make (or order) a custom size mattress for a waterbed insert but most factory direct mattress manufacturers can make custom sizes for their mattresses.
Phoenix