Amazon has the 6-Count Reynolds Kitchens Premium Slow Cooker Liners (13" x 21") for $3.48 - 15% off coupon (first S&S order for this product) - 5% off with Subscribe & Save (cancel anytime) = $2.79 with free shipping on $35+ or with Prime. 

Slow cooker bags are ideal for cooking pot roasts, chicken, casseroles and other foods.