Do you have any old fashioned, local diy shops? That's where I got mine from. The kind of shop where they have screws in cardboard boxes and you buy them 10 at a time.
I got mine at Homebase - £1.99 for 0.5L! It's really worth looking around carefully though - at the Cardiff homebase it was quite well hidden away with the household cleaning products! none of the staff knew whether they sold it either!