You can get away with less quality and cheaper wood with a closed burner because you do not have to worry about the spitting.
In Britain our local newspaper have adverts from people selling wood especially for those type of burners maybe you have to same.
They normall quote price including delivery in Britain maybe they do the same where you are.
I bought wood when the wood burner was brand new, but our neighborhood is about 45 years old now, and SO ARE THE TREES. LOL! Every time we have a storm, there are downed branches which my son chops up and lets stand for a year to dry out the sap inside. Last year he discovered that our parks dept. cuts up broken branches into a size that fits my car trunk, and leave it there. They gave him permission to take it. We use the wood burner in addition to the furnace which keeps the gas bill down. Not that the gas bill wasn't outrageous last winter anyway.
An whenever the neighbors have small branches down, they are delighted to have my son take them and use them for kindling. It works for everybody that way because the neighbors don't have to cut them up for collection.