I'm posting (with permission) something Dr. Welty recently said in email correspondence:
Even if we cannot make an explicit, deductive case for sola scriptura, from the Scriptures themselves, we can make an inductive case. Again and again, in Scripture appeal is made to Scripture to correct oral tradition and oral claims. But we never see the reverse: we never see oral tradition correcting the deliverances of Scripture. So the inductive case for sola scriptura is pretty clear: if you want to know what Scripture commends to us as the ultimate standard of doctrine, it is Scripture itself. It's not like Scripture leaves us guessing here. How is this any worse than our indirect, inductive cases for other doctrines?