It's not dressage saddle shaped, but the Haf pad from
www.goldfinch-endurance.co.uk is very good. It has pockets for extra shims, has a wool fleece underside and airnet material on top, only comes in black atm. I've found it totally non-slip under treeless saddles on the widest roundest cob in the world (IMO!). It's £70 and the shims are about £16 from memory.