If you are just buying a few tree pots, you can get them through Green House Megastore. If you need a bunch getting direct from Stuewe & Sons give you a lot more choices. I think the 4X14 are a good option if you are just growing them one year then planting out. You can fit 9 in a milk crate to make them easier to carry around and to keep them from falling over, etc.
You can plant them directly into the tree pots, but I imagine it is hard to get even heat to them all the way up at the top of those tall pots.
I started 3 pounds of KSU seed last year that I got from Cliff England. Since I had so many of them, after stratifying I put them into 10X20 flats on a heat mat kept around 83-85 degrees. This made it easier to get them all heated and germinating. I simply put a little more than an inch of ProMix in the bottom of each flat spread out the seeds so they weren’t touching and then filled the rest of the tray to the top and pressed it down firmly . The flats had holes for drainage and I kept humidity domes on top to make sure the soil didn’t dry out from the heat mats as well as just to keep the soil temps more consistent. I think I divided the 3lbs of seed into 3 flats (1 pound per flat), but it may have even been 4.
After a month or so the majority had germinated and were sending down good roots - a few were being pushed up out of the soil which made it obvious, but I gently dug a few others out to check them. There was a little bit of a J shape starting when I transplanted them due to the shallow trays, but not enough that it worried me. Here they are being planted into an air pruning bed. I just cut the sides of the trays and folded them down to make it easy to get the seedlings out.
They survived with no ill effects from being transplanted after germination and did great. Here is what they looked like a couple of months after transplanting.
I definitely recommend starting them inside and getting a jump start on the season since they grow slow and will also germinate quite slowly outside in the colder soil. Starting outside some may barely get above the soil before fall.