Drop Zone do the Webtex one for about Â£15. Best price of all shops. You have to search it though, if you go through their site it comes up as Â£20. Random!
I then bought a 40 page nirex-like folder (not actual nirex. a 40 page from somewhere like RV Ops will cost Â£20+, my black one from WHSmith cost about Â£3.50) and it slots in nicely. It'd easily fit another one if I wanted to, or it could hold a ring binder.
I use one and it's quite good. People moan about Webtex, and yes the webbing is a bit shite, but I don't wear my folder for 16 hours a day, drag it through mud and fight with it. I just hold it and read stuff, doesn't need to be Arktis or Black Horse to do that.