If you want a reentrant baritone in C-tuning an octave below, here's two things you can try:

Use the D and G strings from two standard baritone sets: one D tuned down to C (3rd string), the other up to E (2nd), one G at pitch (4th) and the other tuned up to A (1st).

Southcoast sells two specialty string sets for baritone which are like an octave-lower reentrant C-tuning except that the C is at regular C-tuning pitch (an octave higher than you want). For the C string, you could use a normal bari low-D string, as above. Or, Southcoast also sells single strings for a lot of instruments; one of those might fit the bill, though I couldn't tell you which one.

I was searching for a source of single wound low-G strings for a tenor (they wear faster than the other strings), and was told that a lot of folks just use single D strings for guitars. It might be that a guitar A string would do nicely for the C, though I'd worry about the nut slot being wide enough for one. I also could predict how slack it would feel or how the tone would match.