Walmart [walmart.com] has AeroBed Insulated Mattress Pad for Air Beds, Twin, White for $8.00. Shipping is free w/ Walmart+ (free trial available [walmart.com]) on orders $35+
Also Queen [walmart.com]
Product Description from Store
Make your air mattress warmer and comfier with the is filled with disinfected fleece fibers. It has great resistance against unwanted microorganisms and undesirable odor build-up. The soft fleece filling brings comfort to your body by making it feel more like a cozy traditional bed. The air inside an air mattress tends to cool overnight. This is what makes airbeds too cold to lie down during winter. But placing Aerobed Insulated Pad Cover on top of your airbed brings warmth. It traps your body heat, allowing you to sleep all night comfortably. The polyester pad cover is imbued with stain-resistant technology. You can easily wash it in the laundry machine without worrying about spilled coffee or wine forever ruining the fabric. You can reuse Aerobed Insulated Air Mattress Cover with any 18-inch tall airbeds.
Store: Walmart