No problem.
They're sold as 'space boards' by the way - designed for loft insulation. So that's the aisle you'll find them on.
Not that I can guarantee every branch of B&Q carries them. But I got a couple yesterday at the Crawley, Sussex, branch - which is a medium-sized shed outlet, not one of the huge depots. And they had a stack of them.
Interestingly, I noticed from a post over on GWP that Focus DIY sell the same product as well at £24 a pack, and currently have a 'buy one pack get a second pack free' offer. The only problem is (having looked on Focus's website) it doesn't actually tell you how many sheets you get in a pack!
If it's four sheets, like the original B&Q packs, then that means you can get 8 sheets of the stuff at £3 each, which is even more ludicrously cheap than B&Q.
If there's only two or three sheets in a pack though, then B&Q still looks like the best bet.
Either way - get 'em while they're hot...