There is a company that makes a programmable power supply where you can = tap off about 3 different voltages from one power pack (set of = batteries)
Unfortunately I couldn't find them again on the web. But they did have = ads in Robot magazine and probably Nuts'n'Volts, too. The devices were = pricey, though, from $50 to $200 or so. Depends on your budget & needs. = Separate batteries would certainly be cheaper!
Not much help I guess but if you flip thru Robot mag you might see who I = am talking about.
JCD