You can always get them on the cheap, but what you will get will be shite.
I have bought A4 and A5 nyrex Webtex brand and they cracked, and OHP pen stained them pemanently! Utter Shite.
As Parky Boy says, HM Supplies...
They do ones with brass screw keepers, so you can move stuff about in the file without the nause of taking it out of the clear pocket. I've bought loads of extra pockets in A4 and a even more in A5, as I put so much of my instructional material into nyrex. HM also do a nice line in A4/A5 zipped covers too. Well worth the money!