I have a G. passier & Shons jumping saddle in a narrow fit, which has been very good on my skinny high witherd TB. It's a 17'' seat, but i'm only 5'2 and have been fine in it. He has started to muscle up though and has just out grown it... so it's looking for a new home!
My new sadle is WOW which is also great on his high withers, and i'm hoping that i won't need to change it again as it should adjust to fit him.
I have just bought a Henri de Rival 17" narrow from e bay very cheaply and it is in excellent condition. Fits the pony very well and is a dream to jump in. Its my first jumping saddle but I will always jump in one from now on - it makes such a difference to both the rider and the pony!