i got mine in a Robert Dyas about a month ago. It is also in wilkinsons. The one i have is in a white bottle with a blue 'home care essentials' label. about 4 quid.
I got mine from my small local diy shop. It's Jeyes Kleen Off ammonia. Since the bottle doesn't say, I emailed the company and asked - it's pure ammonia, 9.5%