You can always buy plain egg cartons new. There are several online sources. Just google "egg cartons" and you should find them. They tend to run $40-45 for 125 boxes, with free shipping. Misprints may be slightly cheaper but they would typically have other names on them that you're not authorized to use.
Egg carton labeling is one of many reasons why I give eggs to friends, family and neighbors, rather than sell them. Selling gets too complicated.
And a lot of the feed stores sell them too.