Drop-tuning with a set of standard-gauge guitar strings tends to lead to unstable tuning and ropey intonation; when you drop low, you need to go heavy, and DR Strings' Drop-Down Tuning sets are specially designed for the job.
Claiming "accurate and clear lower pitch notes", Drop-Down Tuning strings require minimal adjustment to lock in tunings and intonation - apparently, some players have called them "so stable it is almost eerie". Not as eerie as that spider on the packet, though - brr.
Provided you're not an arachnophobe, Drop-Down Tuning strings are available in 11-54 gauge Extra Heavy, 12-60 gauge XX Heavy and 13-65 gauge Mega Heavy sets for £9.10 each.