Does anyone know where to purchase a Triple Expansion Marine Steam
Engine? I have seen sites that sell kits to build your own, but I
would prefer to buy an existing engine. Any help would be appreciated.
Thanks.

Sometimes you can find them here

http://www.pcez.com/~artemis/NWSSclassified.htm

You might try a "Want to Buy" Ad if you don't see anything you like.
Be sure to specify horsepower, as triple expansion engines have been
used for everything from launches to battleships.