You mean the black plastic cases? Most of the time you have to buy several in a pack but you can get them online:
25 DVD Cases for $13 (link goes to Amazon). As an alternative, office supply stores carry them and might have them in smaller packages of five or ten. Seems like the last time I bought them that's how they were packaged.
EDIT: It just occurred to me you might mean the clear, square boxes like music CD's come in and those are called "jewel cases". They come in standard and slim configurations. These can be had at the same locations as above, you just need to know what to ask for.
25 Standard Jewel Cases, $13
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