For perspective, when you tune up by 3 semitones the tension rises to 141%.
You need a set designed for drop C tuning. Some shops sell single bass strings, or you could find somewhere online to mail order a custom set of single strings (such as
Build your own string set!). Or Circle K Strings sell 'drop tune' sets here:
vStandard Drop-tuned 4 Strings, they're designed to have equal tension strings in a drop tuning.
To find your ideal gauges use the tension chart for 'bass guitar - XL nickelplated roundwound - long scale' here:
http://www.daddario.com/upload/tension_chart_13934.pdf
Use the chart to find your ideal tension, then use that tension value to choose gauges for CGCF.
Example: If you like a 100 E, that's 36.5 pounds of tension, so choose either 125 80 60 45 or 130 85 65 50.