At $6 to $10 per speaker depending on thickness, this is a great source for speaker grill foam. Be sure and scroll to the bottom of the page to find the speaker foam. I was able to get 4 grills for my DR280 from one piece of 72" X 48" foam for a total of $37. (Keep clicking on the link- it eventually works!)
http://www.foambymail.com/Hi-FlowFoam.html