you can buy it from a supplier in Cumbria: I guess if you are close by you can buy the odd bag or two, but when I enquired about a delivery down here (Gloucestershire) the minimum is 50 20kg bags, basically a truck load.
Apparently you only need 4 bags to make a bed and 1/2 a bag top up a week. Sounds too good to be true?
I use Megasorb which I like very much. It really is very absorbant only slight drawback i've found is that it can build up in feet.Solved that by adding Bedmax on top and muck out every day. Excellent in the trailer as well. It costs £5.35 per bag and I get from an agricultural merchant.hope that helps.