TLDR: The answer is Amazon an no it's not an encasement. I kind of want an encasement for the next good pillow I buy but I don't deem it necessary for a mattress. Let me explain.
SafeRest Mattress Protector - Twin Size Cotton Terry Waterproof Mattress Protector, Breathable Fitted Mattress Cover with Stretchable Pockets
Rambling:
I did some research about mattress protection and was willing to spend quite a bit to protect my latex mattress.
All of my product research includes the now pay-walled website called Wirecutter.
Wirecutter had an extensive review of mattress protectors. I simply got the first one after reading through their interviews with textile scientists. Their in-house field testing also influenced my decision.
I use Wirecutter for almost anything as I just moved and decided to pay for a year subscription. Even for silly stuff like dish drying racks. But when I pay for a service, I try to get the most out of it. Plus, it satisfies some kind of shopping or maybe hunting urge. It's relaxing to me to over-research stuff.