My money is on Crust. There was a high profile case in the States some years ago involving Brembo in a liability suit. Since then, no road spares are available from Brembo. The race spares on Yoyodyne's website are readily available.
If i remember correctly you could buy seals of a guy on ebay he would make them up for any bike all you had to was give him the sizes of the seals. And couldnt you have new pistons machined?
I spoke to Steve Tuckey at GPR last week about this, his take was they never sell spares as they've had too many problems with bodged kit, but they keep the servicing costs really low (non-profit on the labour) to encourage people to send stuff in.
It's also nice to know that the sames guy working on your kit prepares many of the WRC/GP2/GP Masters/BSB/Le Mans teams brakes. Got to be worth the hassle.