I bought a mattress protector from IKEA for $25 which goes over the mattress and the topper. It is 80% cotton, 20% polyester with Polyurethane coating to make it waterproof. It is machine washable. Search for SKYDDA HÖGT on IKEA. The only issue I have is with the fitted sheet which goes over the protector--this keeps on coming free due to the extra thickness. I will have to look for another that is suited to thicker mattresses.
My mattress cover was deep and it covers my topper and my mattress both. Our mattress is 3 years old and pretty thick with a pillow-top. It still fits our mattress even with the topper. The topper is 3" thick.
Measure the depth of your mattress and be sure to get a waterproof mattress pad that covers both the mattress and the topper, as any spills on the topper will be impossible to get out! We had to purchase new sheets and new mattress pads after buying this topper, as our mattress was now very deep. But definitely worth it!