@suitesleep has a good cotton knit mattress protector. They are a trusted member here. You can order direct from them; we have it too here. I personally use their washable wool pad on our latex mattress and like it very much. Suite Sleep may be able to give you more information and recommendations on both of those.
Obasan makes a very nice cotton knit protector that is stretchy. This cotton pad, a store brand made for us, is also stretchy.
If waterproofing is needed, these organic protectors from Naturepedic or Sleep and Beyond would fit the bill.
Here’s an article about mattress protectors that I wrote for this site - perhaps it would be helpful.
Good luck to you!