I have bought some from F. W. Schumacher in the past.

http://www.treeshrubseeds.com/

Here is a link that might be useful: seed

Bought some other seed this year from this company a few weeks ago. They got here within a week.

Arktrees

Here is a link that might be useful: Sheffields

I got mine from F. W. Schumacher. Ended up with too many shantung maples. Be careful with damping off fungus though.

I've had best luck if I put seeds on slightly wet paper towel in the plastic container with the lid on and leave it in the fridge. Most germinated within 2 weeks. It was pretty easy and best way to get rid of nonviable seeds. I'd do it in March so keep seeds stored dry in the fridge till then.