Beavercreek Nursery, in Powell, has some seedling (or not much larger) trees. They used to have a really good selection of fairly inexpensive small trees, but seem to be moving more towards the traditional nursery size plants. I never bought a ton of stuff from them, but I still miss their previous selection.

There are MANY good mail-order nurseries for seedling trees. I think you are probably looking for specimen trees rather than mass/reforestation purchasing. If I'm wrong, let me know, and I can fill you in on the suppliers for that type of thing. As for individual/specimen tree suppliers, here are a few:

Oikos Tree Crops has some interesting stuff. Much of their stuff is grown and delivered in plant bands. Some come with really nice root systems that are easy, with only minor manipulation, to prepare for planting. Some come with a root system nearly beyond fixing, due to being in the band for far too long. I still buy stuff from them quite a bit, but I always keep in mind that their stuff is of varying quality (so far as the plant's root systems go). The good news is that prices are low enough that it's often worth the gamble.

Sandy Mush Herb Farm is another one of my favorites. Just by their name, you might not think of them as a seedling tree supplier, but they'll surprise you. They have quite a bit of stuff and at WONDERFUL prices. I think every plant in my last order was nearly perfect. You should definitely check them out!

Nearly Native Nursery is another good one. I think their prices are very fair and their quality was very good. I have been very happy with my orders from them.

There are at least a couple of dozen others, but those three will get you started.

Another SPECTACULAR source of tree seedlings is the East Tennessee Plant Swap. It is held twice yearly, and we always have LOTS of tree seedlings to make people take home (I mean to give away to good homes). Seriously, I grow a LOT of small tree seedlings, and appreciate people at the swap being willing to adopt some of the extras.