Colgate Bassinet Mattress Foam Pad with Waterproof White Quilted Cover, Rectangular, 18″ x 36″ x 2″ Price:  $29.99 (as of 18/10/2017 13:05 PST- Details)

18″ X 36″ X 2″ rectangular shape cradle mattress fits many brands and models of cradles
Extra comfort and support provided by 2″ thickness and can be used on two sides
Waterproof and tear-resistant white quilted cover is easy clean with damp cloth and mild soap