If you are looking for something in which to keep your organizer in order to protect it from keys, hairbrushes and other sharp things in your handbag, I have compiled a list for you.
Firstly, you can look in any shop which sells toiletry and makeup bags as they often have really nice ones in various sizes. These are often also waterproof (good in case of spills).
You can also look on Etsy for Filofax covers, Filofax pouches, Filofax sleepers. I have found the following ones (links to the shops are in the captions)