There are electric heated cuddle cups, and mats available. Some electric mats are rated for outdoor use, so you can use them in outdoor shelters or on a porch for feral cats. There are even electric pads made to fit the igloo outdoor pet houses, which can make life for feral cats better in the winter!
Heated Cat Pad - Indoor Luxury Bed for Cats - 1800PetMeds
are polyester mats with an electro-static charge that traps hair, dirt, and dust. Fibers work as an insulator to absorb and hold heat. These would be good to put on a stuffed chair a cat likes to lounge on.
Heated Cat Beds and Pads | CozyWinters
Heated Pad For Cats | Heated Cat Pad | CozyWinters