Check with an orchard for any limbs they might have pruned or fell naturally. While not blanks per-sey they would be a source of the desired material. You might have to wait until the right season though.
PSI sold them a few years ago made by a company called Harvestwood blanks, but I also remember them being on clearance not too long after. I ordered a few and was never able to complete a pen with them. They always fell apart. Maybe a store that carries PSI products might have a couple left in inventory.