There might be a number of them, but this is the one I know is highly regarded. I'm not sure if this is small enough though. http://www.google.com/products/c... http://www.best-juicing.com/smal...
Besides the obvious, yellow onions are generally the cheapest, and have the strongest flavor but cooking mellows their flavor. White onions are a little bit sweeter and less pungent.
Nutrition wise, there's very little difference.
Sweet onions come in yellow or white (and a variety of other colors), and are distinguished from other onions in that they have a much higher water content and a much lower sulfur content.