Question 1: Where is Sola Scriptura in the Bible?
Short Answer: John 20:31 says, "But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name." And many other verses.
Brief Explanation: John's statement implies that a person could pick up John's gospel, read it, believe it, and receive eternal life in that way. Moreover, John's statement at least hints at the fact that the other gospels have a similar purpose - they are written for us to read, believe, and have eternal life.
Possible Objection: But where is the only in that text?
Response: The sola or only of "Sola Scriptura" is simply a negative claim - in other words, it's saying that Scripture is unique - there's nothing else like Scripture. If you want some verses that emphasize the unique character of Scripture, those also exist.
For example, Romans 3:4 says "God forbid: yea, let God be true, but every man a liar; as it is written, "That thou mightest be justified in thy sayings, and mightest overcome when thou art judged.' (Psalm 51:4)" This emphasizes the crucial distinction between God's word and mens' words.
Another example is this:
Deuteronomy 13:1-5If there arise among you a prophet, or a dreamer of dreams, and giveth thee a sign or a wonder, and the sign or the wonder come to pass, whereof he spake unto thee, saying, "Let us go after other gods, which thou hast not known, and let us serve them;" thou shalt not hearken unto the words of that prophet, or that dreamer of dreams: for the Lord your God proveth you, to know whether ye love the Lord your God with all your heart and with all your soul. Ye shall walk after the Lord your God, and fear him, and keep his commandments, and obey his voice, and ye shall serve him, and cleave unto him. And that prophet, or that dreamer of dreams, shall be put to death; because he hath spoken to turn you away from the Lord your God, which brought you out of the land of Egypt, and redeemed you out of the house of bondage, to thrust thee out of the way which the Lord thy God commanded thee to walk in. So shalt thou put the evil away from the midst of thee.
The point to take away from that passage is that even if someone has authority that appears to be attested by working wonders, the person's message should be judged by the Scriptures (in this case, by the Pentateuch).
Paul similarly warns the Galatians: "But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed." (Galatians 1:8) Someone may object that "preached" could refer to the gospel Paul delivered orally. Nevertheless, we have that gospel in written form today.
Likewise, the Bereans are commended for subjecting the apostles' own preaching to a comparison with the Scriptures: "These were more noble than those in Thessalonica, in that they received the word with all readiness of mind, and searched the scriptures daily, whether those things were so." (Acts 17:11)
Question 2: Where is "Scripture interprets Scripture" in the Bible?
Short answer: 2 Timothy 3:16 says, "All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:" If that is true, then it follows that all Scripture has one divine author even if it has many human authors.
Longer answer: Indeed, we have examples of Scripture interpreting itself explicitly, such as the quotation from John 20:31, above, which provides a purpose for the book of John, and more broadly for Scripture. Other examples include the citation of Old Testament passages in the New Testament, together with explanations of what they meant or how they were fulfilled in Christ. Indeed, sometimes the New Testament includes Jesus' own explanation of his parables. Numerous other examples could be provided.
Rejoinder: But even if we had no answer, can the matter seriously be doubted? Does the person asking the question really think that the Bible is either incomprehensible or should not be understood by taking one part in relation to another?
Even the Roman Catholic "Catechism of the Catholic Church," puts it this somewhat poetic (and consequently imprecise) way (CCC 102):
Through all the words of Sacred Scripture, God speaks only one single Word, his one Utterance in whom he expresses himself completely: You recall that one and the same Word of God extends throughout Scripture, that it is one and the same Utterance that resounds in the mouths of all the sacred writers, since he who was in the beginning God with God has no need of separate syllables; for he is not subject to time.
We understand that Rome wishes to deny Christians the ability to judge her doctrines by Scripture, but surely it cannot be denied that Scripture does interpret Scripture. How else would one read it? As just isolated statements each possibly meaning anything at all? The very notion seems bizarre.