CPF member Warren sells them here (JiLITE brand). You can also use a pill carrier available at most drugstores. There was a discussion about this here recently, do a search and you should find it easily.
I use a plastic pill carrier. Not all pill carriers fit even if they look like they do on the outside. I bought 2 from a local CVS and had to return them because they were a couple of mm too short on the inside due to the cap's recess not being large enough to fit the 123A battery. Look around and you should find some that fit from a local drug store. I believe there are some low priced plastic and aluminum ones online also, maybe amazon.