Boiling onions are found world wide. They are harvested when the plant has  formed 1 to 2 " bulbs. Boiler onions are very small white or yellow storage onions, usually about two inches in diameter. They should be firm; have short necks; dry, papery outer skins; and be free of
blemishes. They come in yellow, white and red, and are used in stews, or served boiled and pickled.  

Here is an onion site to link up to for more onion information:

http://www.sunspiced.com/oconsumer.html

http://www.onions.org/