I haven't ordered yet, but I found them here:
Ventilated Super Sack ョ Containers
I can get the non-ventilated bags free from one of our plants, so I'm going to grab a bunch and play with them to see what I think.
Here's an update on this.
I grabbed some bags from our plant. They have a chute on the top and a chute on the bottom, with string closures at both ends. The bottom string closure has a quick-release clasp. There's a flap on the bottom so that you can tie the chute closed and then cinch up another string to close the flap. That one also has a quick-release on it. These contained food-grade material, so there was also an inner plastic bag. I just pulled those out and tossed them.
The chutes are useless for firewood, so I closed everything up at the bottom. At the top, I cut around three sides at the edges of the cube to make a big flap:

Then just fill them up. I don't fill them to the very top so that there is extra material to close them off with:

And set them on a pallet:

Then, I pulled all four lift straps together and through the top chute. Then I tie the top chute tight around them. Now I can move them around with the tractor.
So far I'm really happy. I will be able to split and toss the wood right into a sack, then I can move my split wood around anytime I want to. When I want to give a load to a friend, I just set the pallet in my truck or on my trailer and away I go.
We've had a few heavy rains and there is no evidence of water in any of the sacks, so my closure system seems to be working.