where can you get one of these

I've seen them listed on aquabid occaisonally. They also turn up among shipments of "ghost shrimp" occaisonally; when they're small/juvies they're quite difficult to distinguish from other species. The main difference is, most "ghost shrimp" don't keep growing, eventually becoming large enough to eat most of the fish in the average "community tank".
Here's a current auction for a related species: http://www.aquabid.com/cgi-bin/auction/auction.cgi?fwinverts&1244589009

Macrobrachium rosenbergi... mean lil buggers