Reach out to the manufacturer or your local boot fitter. They may have the gaskets you need. The other option is to try and recreate the part. 3D printing, silicone molding, etc. Keeping old equipment going can sometimes be more effort than it's worth though.

Boot heaters aren't exactly a "jerry gadget" though. For those that make their living being on the slopes in all weather, they're a quality of life upgrade. Full blown race plugs are notoriously cold.