The single large cases are a universal design. They will hold either two 18650's or four 18350's or 16340's. They could also be used to hold two 18490's or 18500's as well, with some dead space at the end. Unless you want something really small, personally I just stick with this size as they can hold anything. They do make 2 battery cases for 18350's or 16340's - but that's all they hold - literally.
The harder cases to locate will be for the 26650's, or specifically for 18490/18500's if you want something that's an exact fit. Honestly? Check some empty pill bottles as they are a good size for 26650's.
DX.com (DealExtreme) has a lot of battery cases for low prices - if you don't mind the 4 week slow shipping time from China.
As mentioned, RTDVapor definitely has them. Good Prophets as well.