Even if they don't have them at Lowe's or H.D., they can order them for you. They're not available in all regions. They don't carry them in northern NJ by me, but they will order them.
I had the same trouble finding these things, but I was in the wrong section apparently. They are in the building supply area with the drop down ceiling tiles.
They were more expensive than I expected (~$12) and I would recommend Home Depot over Lowes since the ones from Lowes were about halve as thick.
