Amazon offers the Sleep Better 2" Visco Twin XL Memory Foam Mattress Topper, model no. 51581, for $35.25 with free shipping. That's $6 under the lowest total price we could find for any 2" Twin XL memory foam mattress topper sold elsewhere. Sales tax is added where applicable. It measures 37" x 77" x 2" and is made from visco-elastic open-cell memory foam.
Update: Note that this item is temporarily out of stock, but can still be ordered at this price for delivery when available.