There are a lot of good brands that work with high withers. There are several tack stores that advertise on line, even though they have actual tack stores, and they will tell you how to do a wither tracing and submit it to any of the stores offering this service. It is best to have a saddle fitter come out, but if that is not an option, these tack stores can help.
I know that Pelham Saddlery as well as Trumbull Mountain show how to do wither tracings and will offer help over the phone.
Hope this helps.