if your regulator doesn't have an integral one (micromatic regulators do) then you can get by in a pinch making one from the lid of a gladware or other disposable storage container type thing, cut a washer out of that thin plastic and it should work fine.
I got one at Ace Hardware once. Otherwise it's online brewstore orders.
I went by a couple of ACE stores and the only washers they had were close in size, but the opening is bigger. The normal size washer is 3/4" outer diameter and an inner diameter of 1/4". ACE has 3/4" OD x 1/2" ID. Do you think this will make much of a differnce?