And the Catechism can be found on the Vatican website (Link to Catechism). No reason for us to hide it from anyone, nor to hope that Protestants don't read it.
Nor is "because we said so" a good reason to claim that Sola Scriptura is illegitimate. That the idea is foreign to the first 1200+ years of Christianity will suffice.
Trouble with that is, your reasoning is circular. The reason the "first 1200+ years" of Christiantity are said to have rejected sola scriptura is because Catholicism, which actually developed at a very late date, "says" so. Hence, Catholicism's assertion is self-fulfilling because it depends on itself to be true.
A little reading of history refutes this.
We can always use the Scriptures themselves to show this anti-Scripture alone argument is false, but if you are a member of a church that holds other things like "tradition", or a group of theologians meeting together, as equals of Scripture that won't do. We do have the sketchy history of the many pre-reformation independent Christian churches that held that Scripture was the final authority.
Also, I came across this tidbit in Miller's Church History:
Clement of the Scotch Church was condemned as a heretic by a council at Soissons in March 744 AD. He was condemned for holding the view that "no councils, writings, decisions of the church that are contrary to Scripture had authority over Christians".
So history shows us the idea of Sola Scriptura was not foreign prior to 1200 AD.
So, Jesus was wrong when He said "It is written...."?
HE seemed to think that Sola Scriptura was sufficient.