The only flat six-string sets I'm aware of are the TI Jazz Flats JF346 and Rotosound Jazz Bass flats RS776LD, both of which
AFAIK are only available in 34" scale length. They may or may not work on your 35" scale, depending on the actual string length.
Both the TI and Roto high C's are available separately from JustStrings.com. The sets are there, too. They also have the D'Addario Chromes .032 high C string 34", which are not in any set.