AA batteries are, by far, the most common alkaline cells, and that’s the size you most likely have lying in your junk drawer.
Not that many devices use C cells, so they are much less likely to be lurking in the corner of that drawer.
AA’s and C’s are the same height and the same voltage.
A clever Lifehacker tip shows you how to wrap a little cardboard around a AA battery so it will fit into the slot intended for a C cell.