This isn't quite as fancy but it will save you a lot of money if it works for you and you are willing to try it.
EVERY SINGLE TIME I RIDE.
I use a thin cotton pad, that's what I put on first. Then, I fold a plain white leg-wrap in half long ways and put it over her withers. The saddle goes on, and she's got wither support. It isn't a large piece of material over the already-thin pad which is great for the heat or just not to have that much extra material there.
If this method works for you (my saddle fitter showed me and it works perfectly every time), you can purchase and mix and match a bunch of thin and cheap saddle pads and get a leg wrap and then you are done.
If it doesn't work out, there are some different wither relief options and pads. Check out Dover Saddlery. They also have a great online chat service and the live reps will walk you through some good options based on your situation -- they are educated so feel free to share your issue with them.
Thanks and good luck, happy riding! (:
God bless, englishaqh (: