I play dropped D primarily these days and with all types of strings I dont notice that much of a difference.
Dropped C however is a challenge. I am assuming you are talking about a traditional CGCF tuning (Low to high). If so, what worked for me in the last band I was in was this:
Ernie Ball Slinky's 5 string set.
Throw out the E string and use,
The B string in place of the E.
The A in place of the E
The D in place of the A
and the G string in place of the D.
A minor setup will be required (Reslot the nut and tweak the action a bit) You will be able to bring the B string up 1/2 step for the low C with less "flop" and detune the remaining strings one whole step.
Both Pacman and Rav's suggestions will indeed work, but my preference was to use the 5 string set minus the E string...it felt better to me and there was more tension to the strings in my opinion.
This is, bearing in mind, you were in fact talking about a "Dropped C" tuning (CGCF)
Hope this helps.