The Napa Cabbage is really Chinese Cabbage (from China) and dates back to the fifth century A.D. It is believed to have been a naturally hybridized cross between pak choi and a turnip. The plant did not show up in Japan until the late 1800's but Japan has since produced many hybrids.
Napa Cabbage is produced in the US primarily in California and Florida to the tune of over 12,393 acres per year which is why it can be found almost as easily as the common green cabbage.
There are two varieties most commonly found in the US. The most common (show above) is "Napa" cabbage and is stockier with crinkled outer leaves - dark at the top and white towards the stem-end. The other type found in the US is called "Michihli" which is long and slender with ruffled top leaves.
Napa Cabbage stores well refrigerated. Do not wash it first and place in a plastic bag. Store in the vegetable crisper for up to 2 weeks. When it is at its freshest use it raw in salads but as it gets older keep it for cooking only.
Napa Cabbage can be found in most well-stocked grocery stores as well as in any Asian market or you local farmer's market. Look for solid heads with no browning on the tips and leaves should have a nice sheen to them.