I'm sure you'll find some, it's pretty common, would've thought P@H should do it? Pretty much any pet store should stock it for around a fiver for a huge bag. Most supermarkets should have some in stock.
I noticed the Scumerfield near me don't stock woodbased litter - I think ASDA may stock the Bob Martin version (I hate BM tho but at least their wooden cat litter can't be that evil though what they do with the proceeds may be)
As I say it may seem pricey for a bag of stuff your cats going to sh** in (and I'm on a tight budget myself so I know what I'm on about) but it seems to last for ages compared to the basic stuff. And I bought some freshener but I'm very doubtful about it - it says do not flush or compost, it can harm aquatic life etc - so what does it do when the kits lick it off their paws???
Well if it does woodbased does work out more pricey for you you could always just use it for the indoor tray. It can be a tiny bit messier than the heavy stones but really is so much nicer to have indoors x Trust after your first bag you'll never look back!
I know some ppl use sawdust - very credit crunch agreed, but I can imagine that making a right mess!
Good luck xx