63-66 A-body convertible rear seat and brackets

First the bad news. The bottom frames of the verts rear seats didn't have good survival numbers. Rust ate them up. I'm not saying you'll never find one but what you find may not be worth the shipping cost.
Now the good news... The frame you need isn't a heck of a lot different from what was used in other models ( be glad it isn't the back of a vert rear seat you need ). The bottom frame you need can be created with a torch and welder.
Some companies like Legendary have done this sort of thing but at a hefty price. The local upholstery shops farm out the metal work.
Their added cost for that is the main reason my bro-in-law is doing it himself.
His vert is a 66 Impala and his donor is a 68 Caprice.
Funny thing is , he bought the Caprice for its BB 396, front disc brakes, etc.. and hadn't even looked at that rear seat bottom.
I compared it to Cinderellas glass slippers. He had what he needed , just didn't know it. LOL