If your saddle is wide enough, but sitting on the top of the withers, these work perfectly well for the short-medium term: http://www.doversaddlery.com/roma-wither-relief-lift-front/p/X1-19456/
You can get them padded up in the front, padded up in the back, or with neither (my preference). I used one of these on a shark-finned TB, and it did the job. Of course, if you pad up the front of your saddle, you may end up sitting off balance, keep that in mind.
If your saddle is nearly perfect, and your horse is in work and muscled, the little shimmed pads like Mattes are great. But if he’s a serious coat-rack who will change shape enormously, I’ve found they don’t help enough. I would usually use a foam pad with a newly OTTB, and then go back to a fleece halfpad once they muscle up some.
If your saddle is too low, and also narrow and wanting to pinch the sides of the withers, then no pad will help, and you’ll need another saddle. Is why it’s always good to have a saddle with a nice wide channel and MW-ish gullet: you can often pad up to get wither clearance, without causing pinching.