You can buy Ninja Spirit
from the Mac App Store right now for $10. Well, OK, it's part of the Irem Arcade Hits
collection by DotEmu, which also throws in games like Kung Fu Master, Vigilante
, and Hammerin' Harry
-- 18 in all -- but all you really need to know is that this is a convenient delivery system for Ninja Spirit
, a truly excellent side-scrolling ninja game.
The Mac version is a new release, but there's been a PC version available for some time now -- if that's your platform of choice for arcade collections, you can buy that direct from DotEmu