Most strings can be bought in sets (9-11-16-24-32-42), leaving us two or three unused strings lying around - i found one or two sellers on Ebay who sell strings individually or in packs of six, so no more unused string gauges in my spares box!
I normally used the following gauges on my 25.5" scale guitars: 11, 16, 24, 32 (b, G, D, G)
I found this "middle four" arrangement from a standard set of Ernie Ball Super Slinkys worked well but was left with spare 9 and 42 gauge strings, the 42 being far too tight to use and upset the apple cart as it were, I managed to obtain 6-packs of the string gauges i normally used but the B or "melody" string seemed far too quiet in the "mix" so tried some thicker strings to bring the notes out.
Another 16 was far too tight (this is turning into a '3 bears' story...!) but made an amazing difference, though made bending a little difficult, so a 14 gauge was brought in and did the trick - nice loud melody string, it matched the tautness of the other strings and was easy to bend!
So my new configuration is.... 14, 16, 24, 32 (b, G, D, G)...sorted!, ...the 24 gauge can be bought as wound or plain but i prefer wound...(-:
FWIW D'Addario (electric, acoustic, classical, other) and La Bella (electric, gut/classical) sell individual strings directly on their websites. La Bella also offers some interesting strings for Eastern and folk instruments including viol and Viola da Gamba that could be of interest to the more experimental builders out there.