Honestly, with Thorowgood you probably don't really need the high wither. Get yourself a flexicurve from any store that sells drafting/craft supplies and do a wither tracing. You might find he would fit into the regular T8 GP just fine. I just got a T8 jump saddle yesterday. They no longer make a high wither and I figured I'd be sending it right back, but it seems to fit with plenty of clearance despite my having ordered the wrong gullet size. My other T8 dressage that is a high wither model, has so much clearance you could put a baseball in there. Big time overkill.
If you want some other options, try: Happy Horse Tack
Trumbull Mountain Tack shop (Just google it) Flying Star Stables Brighton, Colorado : Home
in CO 303-655-1815