I use a standard Halfords 10mm sparkplug socket 3/8 drive no problem. the socket can be awkward to get out but Halfords now sell lockable wiggle bars, they stop the detent spring working so the socket cant come off.
My GS came without the spark plug socket, so I bought a couple of BMW items - 1 for under the seat & 1 for the garage. When I came to use them, I found they were next to useless as they only have a hole through the other end to put a small bar through for leverage - no way or torqueing them up
I then followed the advice from the Adv.Riders site - cut the end off & welded a 17mm nut on - good idea from johnjen.
Why a 17mm nut - this allows you to use the wheel nut tool under the seat.
Why shorten it - shortening it by 15mm approx & then welding a nut on keeps it at standard length & still able to fit beneath your seat.
I'm sure B&W parts aren't THAT expensive - but the really cheap way to do it is to use a jap bike one.
A 10mm spark plug is a 10 mm spark plug - get one from a bike wreckers or rock up at the local bike shop and ask one of the mechanics - theres always a few kicking around - cash talks here.
Thought about the welding thing, but don't do welding.
Get a raw bolt just big enough to fit in the round end of the socket, then tighten it up till it grips and then use it with a torque wrench when tightening the plugs.