A brake shop. Seriously.
Most of the good brake places can get them in.
I use Better Brakes in Bayswater.
Theres 2 types, basically petrol and diesel.
The petrol ones start off life 12mm thick and can go down to 10mm before they/re unroadworthy.
The diesel ones are 10mm wih a limit of 8mm.
(There's also GTI ones which are vented - 22mm thick with a limit of 20mm IIRC)
Make sure you know what type you're after and hop on the phone. You'll soon track them down. Even supercheap could probbly order them in.
Pete
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