Newtone strings do special ‘Drop tuning’ sets which have a thicker core but are the same guage as a ‘normal’ set, basically means that when they’re drop tuned they have the same tension as a standard tuned set. Used their bass sets for a couple of years now, Drop C with a 105 guage (compared to the 130s/135s I was using before) is brilliant!
http://www.newtonestrings.com/
Handmade to order in the UK, usually takes a couple of weeks and slighty more expensive but definitely worth it imo.
EDIT – can see they do a Drop D set as standard would give them a ring, sure they could do it for you!