Go to (google this) "strings by mail".
Assuming that the guitar has straight frets, not fanned, and has a scale of 650 mm, it doesn't matter what set of the upper six strings you put on because every guitar likes different strings, so just buy a few different standard sets and try them.
As far as the extra bass strings go, there are several companies offering low bass sets made specifically for the tuning D - C - B - A
which include Savarez and Aquila, (Aquila are my favorites) and, also, La Bella has a complete range of sizes for long lute basses which
work fine on the guitar but you have to know what sizes you want.
If however, the dimensions or tuning are non-standard or exotic, then you have to go to "Arto's String Calculator" (google that) for the diameters
on your trebles and to the D'Addario site - where you will hunt up a PDF on calculating string sizes - for the basses. The results will only give you
D-Addario basses, but fortunately some of the D'Add basses are labeled with their diameters in inches as part of the part number and those numbers
can be extracted and used for other brands which publish their diameters.
You want to study and memorize the diameters of your preferred strings in both millimeters and inches, as all extended range players find out.
sizes in millimeters range from .40 mm to almost 2 mm for low low basses, always with two decimal places.
sizes in inches run to 3 decimal places, like .020" for very thin up to .070 or so for low basses.
A digital caliper (I have a nice one from Stewart McDonald luthier supply) is very useful.