Probably the wrong time of year now but ASDA and their ilk had cheapo ones for about 7 quid. Works for me, but won't handle a massive grain bill - you'd have no chance of anything over about 5.5kg.
There's also Daab's mash tun - made out of a normal 5g bucket with a few layers of camping mat for insulation. I've heard very good reports about these.
actually now is probably the best time to get them as they'll be cheap! that's if you can find them
apart from the usual suspects like Argos, homebase, the big tescos stores that dont do food (i cant remember the added word in those types of tescos stores hehe), woolworths etc i cant really be of much more help