Well, technically..they do exist. Rare and scarce, they can be found in Australia and South East Asia. They are in the Glyphis genus and most likely illegal to own.
Glyphis fowlerae Compagno, W. T. White & Cavanagh, 2010 (Borneo river shark)
Glyphis gangeticus (J. P. Müller & Henle, 1839) (Ganges shark)
Glyphis garricki Compagno, W. T. White & Last, 2008 (Northern river shark)
Glyphis glyphis (J. P. Müller & Henle, 1839) (Speartooth shark)
Glyphis siamensis (Steindachner, 1896) (Irrawaddy river shark)
Glyphis sp Not yet described (Mukah river shark)
Glyphis hastalis
http://www.elasmo-research.org/conservation/river_sharks.htm