Basically Memory Stick have no development. It's still in production to support the existing devices. I have just bought a Memory Stick Pro Duo (full length) at SSP. It's still available but the price is many times higher than SD and others. My 8G card cost $125!
Also Memory Stick has so many version and there is compactibilty problems of new version of Memory Stick with old version device. Before you try to buy one, you need to check the compaticbilty (just google "Memory Stick Compactibility").