B7major isn't in the key of C. In fact, there exists no chord called B7major. B7 is a chord, so is B major, B7 major isn't a name commonly used to refer to any chord. Is B dim in the key of C? Yes and no, but it's safer to say know. A B diminished triad exists in the key of C, but often when we say that we have a diminished chord we mean a fully diminished chord (spelled 1, b3, b5, bb7.) Since the B chord in the key of C is only half diminished (or minor 7 b5 chord) i don't like saying that there is a B diminished chord in the key of C, even though if we look at only the triads it's correct.
I think it's more useful to look at the 7th chords in a given key than the triads, because it gives you a more complete understanding of diatonic harmony. I won't list them here but I know that I have listed them in the past on this site, and so have many others. You can scroll through old questions to find them.