How can I find a replacement?

Given the age of the fire and the fact that the spare isn't a trivial part or a "consumable", I think you'll be SOL.
You could have a Rey at doing what I did for the in-laws electric flickering flame effect electric fire: I taught myself a bit of electronics and some (really) basic programming for an Arduino computer. I bought some strip board prototyping board, some LEDs, a few resistors, transistors etc, plus an old laptop power supply. I programmed a tiny Arduino Nano to flicker a few of its output pins and used that to drive banks of yellow LEDs arranged across the back, plus I had some red LEDs under the main part of the fire bed.
That completely replaced to power-hungry filament bulbs and fragile spinners.
In your case it sounds like the fibreglass fire bed is disintegrating with age. I'd have investigated replacing that by organising a few "logs", charring them underneath or giving them a squirt of Matt black paint and laying them on top of some crumpled diffuser. The red LEDs should give fire glow and the yellow ones a flickering flame.
The one I did for the in-laws worked superbly and I was well chuffed.

Rjrh
July 2023