I am a physical therapist and see this problem in my practice. If you can find a good PT in your area who understands foot pathology and can fabricate orthotics for you, that would be a good thing. Typically, posterior tib tendinopathy is caused by over-pronation, and a good therapist might also be able to identify other contributing factors. (Weakness, muscle imbalances, tight muscles). I would hope that they would look at your mechanics all the way up to the hip, because weakness and tightness patterns there can also contribute to over-pronation. Regarding over the counter orthotics: I plan to look into those "Sole" ones that were recommended. The only thing about those is that I would want to add some posting to the bottom of your foot before the heating process so that they are molded into a more ideal, rather than dysfunctional foot posture. In my practice, if someone has just mild foot dysfunction, I find that the SuperFeet orthotics help most people. (In fact, that's what I wear). We never really know if orthotics are going to work until we try them, so before I have someone shell-out $300, I have them try the Superfeet ($44), maybe with or without a little posting additions from me. Alot of the time, that's all that is needed, but if my patient needs more support, then we go with the custom orthotics.
You might want to check out my post on "tendonitis vs. tendonosis" to get some more information on the pathology. Good Luck!