Via amazon offers 50% off Waterproof Mattress Protector (various sizes) with promo code W2EHLFER
Twin 39x75inch +14 inch for $11.49
https://www.amazon.com/HollyHOME-…B075FSCFYW
Full 54x75inch +14 inch for $12.95
https://www.amazon.com/HollyHOME-…B075FMN4VP
King 78x80inch +18 inch for $14.95
https://www.amazon.com/HollyHOME-…B075FQZ1RT