Petco offers K&H Oatmeal and Chocolate Self Warming Pet Pad, 21" L x 17" W for $11.46 (reg $16.99) plus $6 shipping.

Product Description:
Unique materials of this mat utilize and absorb the kitty's own body heat, ensuring a nice warm, cozy place for napping. It's the perfect size to place on a chair, bed, or on a piece of cat furniture. It's simple to use, simple to clean, and very affordable. Double-sided with the option to flip over for the desired color on top.

• Self-warming material returns body heat to your pet
• Reversible to match any decor
• No electricity needed
• High quality comfort and warmth any pet will love
• Machine washable, simple care
• One year limited warranty