Try this

Here is a link that might be useful: deep tree pots

If you don't need case lots, try this site for smaller quantities - and they will ship to Canada, but you need to contact them about rates per their shipping info page

Here is a link that might be useful: Charley's Greenhouse

Scottokla's link is broken. Listed below is the new link for Stuewe & Sons. This company carries a variety of tree seedling nursery containers and appears to deal with primarily wholesale growers (but they do indicate they sell smaller quantities - see their website/ordering information).

Here is a link that might be useful: Stuewe & Sons

I bet that sliding a 6 foot long root ball out of a straight sided pipe is an interesting proposition.

I've used the Stuewe Treepots, in the 1,2 and 5 gallon sizes. The problem is keeping them upright. (the 5's aren't too bad.)

The smallest ones, about 4x4x15 fit 9 to a milk crate quite nicely. The #2 will fit 4 to a milk crate. Note that serious production will get the local milk companies annoyed with you as too many crates vanish from behind 7/11s. You can get legal crates teh same size from Shippers Supply.

The other way to support the #2 size is with concrete re-enforcing mesh. Make a 2x4 frame 1 foot high, and use fence staples to fasten the mesh. You can space the pots in various patterns to give them more room.

We would be happy to make you ReleasePots in any size you like. You have never seen anything like it. It opens to perfectly release any size or length root ball. See ReleasePot.com and let me know what you need. I'll get right back to you. US made.