I've noted a number of Roman Catholics who seem to think that the advocates of Sola Scriptura need to prove that Scripture teaches "Scripture alone is the infallible rule of faith and life." I understand (I think) this mindset - if you're advocating "Sola Scriptura" you should be able to prove it. Part of the problem is that some Roman Catholics don't seem to understand that "sola scriptura" is a name for a bundle of doctrines. There are both positive and negative positions within that bundle.
The primary positive claims of Sola Scriptura are that:
a) Everything we need to know for salvation is taught in Scripture. (Sufficiency)
b) Everything necessary for salvation is taught clearly in Scripture. (Perspicuity)
We could summarize these as simply "sufficiency."
The negative claim of Sola Scriptura is that there is nothing else like it. This is a universal negative. But there are also specific negative claims, such as:
c) Teachers that teach contrary to Scripture should be rejected. (Primacy)
d) The Bible does not err. (Inerrancy)
That the Scripture teach (a)-(d) really should be enough for anyone who properly understands Sola Scriptura. The general negative claim of "and there is no other like it" does not have the same kind of burden. In other words, having established that the Scriptures are an infallible rule of faith, we can be content to let all comers try to prove that their supposedly supplemental rule of faith is also one. It's not strictly necessary for us to remove that possibility antecedently.
Indeed, it is illogical for people to suppose that the position of sola scriptura would be defeated, simply because the negative part of the claim were unproven. Until some other infallible rule is established, Scripture alone is the default position.
I mention all of the above without getting into the question of whether the general negative claim can be established from Scripture, but rather simply observing the lack of consequences in the case that it could not. In short, sola scriptura is not discredited until either the sufficiency of Scripture is disproven or some other rule of faith established.