There was a long thread about Ritter Sport chocolate bars on the General Board a few months ago, with many people around the country observing that except for the choclolate biscuit and the milk and dark chocolate versions with hazelnuts (usually available at Trader Joe's), most of the other varieties are hard to find in the U.S. In LA, someone mentioned Bay Cities as having a good selection.
They also sell these at Le Pain Quotidien, but only in the small, overpriced bite size.
As others have observed, this is not the very best chocolate bar in the world, but for the price, not bad at all. There's something especially satisfying about a nice thick bite of strongly flavored chocolate--I have always loved plain old Hershey's Special Dark, in the thick size, for the same reason.
So, anyway, last weekend I noticed a marvelous motherlode of the full size Ritter bars, in numerous flavors, at the Koreatown Galleria Market, of all places. Right up by the cash register, near the kim chee stand, the chestnut-honey-flavored ice-lollies, and the rice cakes that warn that you have to eat 'em in 24 hours or else, that's where you'll find this definitive German chocolate. It figures.