Karen, your post made me think, I have never read about long day onions. I have just bought what is sold around here. The store I buy them from is owned/run by a nice lady and her husband (mostly by her). She sells onion bunches for $2. Some of their stock comes from Dixondale Farms.
http://www.dixondalefarms.com/category/onion_plants
The link above shows the zones for the US. We live right near the line between short-day and intermediate-day, but most people around here grow the short-day type.