I understand some guys here have ordered custom 15mm spacers however there were a couple of issues with that:
1) 15mm maybe too aggressive
2) Those particular spacers required the wheel bearing to be sanded down which to me is a completely unacceptable modification.

Anyone have any ideas of somewhere we can buy custom spacers which are 10-12 mm and don't require modifying the wheel bearings?

 

Lenny will make any size you request:

Wheel Adapters, Wheel Spacers, Hub Rings, and much more! | Motorsport Tech

However, IIRC, it wasn't the bearing but the wheel studs which required shaving. This means if you go less than 15mm, then you will need to shave more of the studs (or replace the studs with longer ones and just have the spacers only be spacers).