These sources have been great for me!

For Oriental Onions!
http://evergreenseeds.com/bunchingonion.html
http://www.kitazawaseed.com/seeds_bunching_onion.html

For common agricultral onions!
http://www.dixondalefarms.com/products.htm

For dividing onions!
http://www.southernexposure.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=ONION

I have also obtained good onion starts from ebay.

As for varieties, Everyone should get a potato onion from someone on their latitude line. Potato Onions will outproduce anything else in your garden. They store well and become life long friends.
Also get a good dividing onion. As a general rule, I would get Evergreen Hardy bunching onions. The right one will give you nearly year round green onions for life.
If you live far enough south, go to :
http://www.nativeseeds.org/v2/default.php
And order some I'itoi's onions. The plants are great, and the I'itoi's I planted a month ago look like established plants already. I have not tasted one yet, but I love a plant that takes off and grows.

Howdy, I was so happy with my order from www.dixondalefarms.com I had my order in 3 days. They have a good web site.You will be happy!!!

http://www.dutchvalleygrowers.com offers one shallot. They have grown it in the area since it was imported from Europe in the 30s. It tastes good, and grows well. They don't have a name for it, it is the shallot that has always done well for them.
-Bob

I second the dixondale farm comment. I received my first order, it was suppose to be a bunch of 60, more like 80. I planted them three weeks ago and everything has established nicely and growing.

I start my onions from seed. The last couple of years I've found "Varsity" a large spanish type purchased from Fedco of Maine, did very well and produced large bulbs which keep well. And "Greek Salad" a large red - last batch purchased from Pine Tree of Maine. Produced huge onions that kept all winter (still eating them!) And I have gravelly soil which is a challenge for onions.

Happy growing!
Maryanne in WMass