Frisket, the rapid disappearance of D cells from store shelves is something that people commonly comment on after various events, which is one big reason why I have tried to standardize on AA for most of my battery operated gear for quite some time now.

However, that does mean that many people may have some spare D cells during an event, so I wouldn't totally throw out all your D cell flashlights, in case you need more batteries and the only thing available from someone are D cells. With the amazing efficiency of LED flashlights now, you don't need big D cells to get many, many hours of usable light nowadays. In the past, you needed big 6V lantern batteries or multiple D batteries just to get some decent runtime before your light started rapidly turning a dim orange.

In many cases, C cells are also pretty available when all the D cells have been gobbled up, so that's another option. But more equipment take AA's than C's so you probably get more interoperability with AA's or even AAA's.