CatsPHiddle, if you played a Tim Phillips 5-string, I can understand. My dream instrument. (If there are any rich people out there who have $3000 they don't know what to do with, please buy me one of those things. I guarantee I'll send a thank-you card.)

I don't know of any sheet music specifically for a 5-string fiddle. If it were me, I might transpose tunes into a lower key that wouldn't be possible on a 4-string fiddle. Of course that doesn't work when playing with others.

I might try playing the B parts of tunes down low instead of up high or try playing low harmonies. I might try playing all sorts of stuff down an octave.

The key of F might be easier since F is where C would be on the 4th string. It's just like playing in G but a string down. I enjoy accompanying songs (when it's called for) and a lot of singers like F. I'm not particularly good at playing in that key on my standard fiddle but I think I'd have a lot of fun with that 5th string.

Now... A cello is tuned to CGDA. A fiddle in standard tuning is GDAE.
Your 5 string has the range of a cello and fiddle combined. Maybe you could try sheet music for cello? I don't think you'll find fiddle tunes, but maybe that doesn't matter to you.