A brick fireplace/chimney will have the fire clay flue liner installed as the chimney is built. The only way to replace the liner is to demolish the brick chimney and rebuild it from the ground up.

Not an easy or inexpensive solution, unfortunately. My in-laws came close to a bad chimney burning their house down and the fix was to install a smaller diameter flexible metal liner inside the existing fire clay liner. A quick search turned up the link below to show you what is available. I suggest you contact several other fireplace shops in your area and, hopefully, find someone more knowledgeable in regards to retrofitting older, failing chimneys with metal liners.

https://www.northlineexpress.com/chimney-liner.html?gclid=EAIaIQobChMIt4z3mdr25QIVQx-tBh2yBgMQEAAYASAAEgLO8vD_BwE

You need to bring in a chimney expert to evaluate what you have and give you options. Even if you had the replace the original clay liner, it may be possible to remove and replace it with a new liner without demolishing the brick chimney. There are tools and methods to do this.