Walmart offers the Comfort Rx 2" Orthopedic Gel Foam Mattress Topper, model no. 1101037, in various sizes from $15, as listed below. Choose in-store pickup to avoid the $4.97 shipping fee. It features liquid gel-infused orthopedic foam and a 5-zone contoured design. The sizes:

• Twin for $15 (low by $7)
• Twin XL for $16.31 (low by $6)
• Full for $18.32 (low by $5)
• California King for $26.43 (low by $7)
• King for $28.63 (low by $5)