I enjoy starting Walnut trees from seed, and Walnuts tend to have roots that go deep fast but don't spread out as fast. So I was looking for pots that are deep but without a large diameter; otherwise, each pot uses more soil than necessary and takes up more room when transporting the trees to permanent homes.
I did not find such pots locally, but was referred to a supplier in Oregon:
Steuwe & Sons (https://stuewe.com/
They have a large variety of pots specifically for trees.
For example, 6" diameter x 12" deep, or 6" diameter x 18" deep.
If you are planning to keep the trees in pots for two years, the 18" deep pots might be better; otherwise, there is not a big difference in price between them and the 12" pots.
However, the cost of shipping can be surprisingly high, especially depending where you live.
Often higher than the cost of the pots themselves, so they recommend ordering at least 20 pots at a time.
Also, they ship by UPS, so if you are not within the US, there will also be a brokerage charge, plus taxes and duties on the brokerage service. Ouch! So, think long-term and, if you order, order a quantity once so you never have to re-order.