I guess it would depend on what type of trees you are looking for. Not to try and feel like a broken record over here though, lol. My first choice would be tradewindsfruitstore.com.

They have got some GREAT choices for trees that bear fruit and nuts. They carry rare trees.

For common trees like dogwoods and maples I don't think tradewinds would be the place, and really I do not know who does carry those.

Few trade trees around the forums and they are hard to come by. I found a person offering a PawPaw tree and I about had a heart attack trying to email them before anyone else could, LOL. Needless to say I scored the trade, LOL!!!

Thanks countrycarolyn, I'll check them out.
I didn't really have anything in particular in mind. Just wanted to try my hand at wintersowing some tree seeds.

Well good luck!! I sowed those pawpaw seeds on december 21st, so hopefully I will have a couple of pawpaws to plant. I will be thrilled!!

The two major "retail" tree seed suppliers are...

Sheffields Seed Company - http://www.sheffields.com/

and

Schumacher Tree & Shrub Seeds - http://www.treeshrubseeds.com/

Another company that has quite a few tree seeds is...

J. L. Hudson, Seedsman - http://www.jlhudsonseeds.net/

All three companies have very good Garden Watchdog ratings.

If you decide to try something these companies don't offer, email me and I may be able to help you out with sourcing.

Wow! I've been checking out the Sheffields site and I had no idea there was place with so many different tree seeds.
If you go to their catalog, under trees, click on Maple for a dropdown of all the different maples they offer - more than 80. It's amazing! Thank you Brandon.
I haven't checked the other websites you mentioned yet.

Brandon, I am so glad to see you on the winter sowing forum!!!

You guys this is who I got the pawpaw from!!

Yes yes yes, thank you so much for the links Brandon! Wahoo! Exactly what I was looking for :)

Carolyn, I pop in here every once in a while. Usually I just read. I winter sow some seeds most years, but still do the traditional sowing form the majority of my seeds. I like to start them early and get a jump on growth. I grow mostly trees and shrubs with a few perennials, especially species iris.

See you need to hang around here more often then. I am sure there are several iris lovers around here. A lot just read, so you mention it though they may pop up, lol.

I am to much of a busy body to just lurk, I got to put my 2 cents in!!! LOL