Not sure where you live but you can buy a waterproof mattress from Sleepy's or Bob's if there is a Bob's furniture store near you. I would go to any mattress store and ask if they have them, I imagine they would be expensive but definately worth it.
I did see some waterproof covers at Kohls. I bought one of the pillow covers for my grandson and it was very noisy when he put his head down and moved around, not the soft fabric type, very plastic.
I bought my mattress from Sleepy's and got the mattress cover and pillow covers included. I would hate to think what would happen to the mattress if urine or vomit got onto it, imagine a sponge that you can't squeeze out...
I was told to turn my mattress about every 6 months, foot becomes head, then switch back, but not to flip it. The foam is a different density on the bottom and my mattress has a non slip type coating on the bottom that wouldn't be comfortable to sleep on even with the cover. Plus, my mattress is VERY heavy, I have a hard time turning it, I can't imagine trying to flip it. It is at least 2 times as heavy as my old pillow top mattress was.