By drop C i assume you mean CGCF?
A .110 or .115 is probably optimum for switching between E and C, 4 semitones of retuning creates a big change in tension so it will be fairly tight at E and extra light tension at C. Check out www.bassstringsonline.com
for single strings, they may have singles in your favourite brand.
I recommend you build a custom set of singles carefully chosen for your range of tunings. The trick is to get all the tensions feeling right in the theoretical average tuning of D Ab Db Gb (assuming your retuning range is EADG to CGCF), this will minimise the tension deviations. If you equalise the tensions in the average tuning that will minimise the tension imbalances caused by retuning. If you can find out the stock strings brand and gauges that will help you choose gauges.