The simplest solution would be to run 2 of your D cells in series.
If you're wanting to only use a single D cell to power them then you'll need to up the voltage, and since that type of LED has active circuitry in them it needs to be clean enough (less ripple) otherwise it's just going to make them reset over and over and not work correctly. There are loads of LED charge pumps, DC/DC converters and the like that will all do that. You could even try out a Joule Thief and see how that works with them.