You can get standard-sized 3-disc Amaray cases at http://usplastic.com/
and the budget ones from http://genesysdtp.com/
. I believe the black budget cases are made from recycled polypropylene, though. If you're not familiar with recycled polypropylene, it has an unpleasant odor, something to consider if you're sensitive to that sort of thing. The white and clear ones are made from virgin polypropylene, so they won't have that issue.
Personally, what I do, is buy 2-disc Amaray cases and replace the flip trays with ones from Versapaks.