Woot offers TETON Sports XL Sleeping Bag Liner for $7.99 free shipping with Prime. 

Product Description:
Enhance your sleeping bag's comfort and cleanliness with the XL Sleeping Bag Liner. This oversized liner adds an extra layer of dependable warmth and keeps your sleeping bag clean, extending its lifespan. Made from soft, breathable fabric, it provides luxurious comfort and is machine washable and dryable for easy maintenance during outdoor adventures. Perfect for use in varying temperatures, it's a versatile addition to your camping gear.