How big is your setup?
I don't think there's much info because we don't keep shrimp that big!
Hey, speak for yourself! :icon_wink
How many do you want?
Aquaculture of Texas (for instance -- there are many other suppliers) has everything from postlarvae to broodstock for sale, but might not be worth it unless you'll be buying in quantity. Occasionally they show up for sale in petstores (usually as "blue prawn" or something like that -- but be aware that a rather smaller South American guy, probably
M. amazonicum or close enough, seems to be showing up in the U.S. semi-regularly under that name these days). mordalphus' suggestion might be best if you're just interested in rearing a few.
If you're not already aware,
M. rosenbergii has extended marine larval stages -- they're incapable of completing their lifecycle in a purely freshwater setup. The
M. cf.
amazonicum coming out of Ecuador that I mentioned above might actually fit the bill better -- although their eggs don't hatch into benthic "miniature adults" as is the case with redclaw Macros or
Neocaridina or so on, they do have partially abbreviated development (with relatively few floating larval stages) and no elevated salinity requirement. And they
are aquacultured for human consumption and bait in South America.