Hardware store, ask for a pair of washers for a 9/16 bolt. If presented with a choice of thickness, you can probably go for a thinner one.
Given the number of bikes not using them, I'd say they're NOT required. With the possible exception of on CF cranks.
OTOH, unless you're concerned about the slight increase in pedal spacing, I can't see them doing any harm either. And if used from the start, will save your cranks some scrapes and gouges.
Last edited by dabac; 05-09-15 at 10:26 PM.