I get the preprinted, generic cartons from eggcartons.com. I also bought date and grade stamps from them. I bought a custom, three-line stamp from an online stamp company with our business name, address and organic certification info that I stamp the cartons with. They look very professional.
Some in our area use the generic printed cartons and then put small printed labels with the required info and hand write the dates, but I figured that would be another ongoing cost for the labels, plus the time it takes to print them. The stamps and ink that I bought were a one time purchase that I figure will last for tens of thousands of cartons. (We are selling 200-300 dozen a month).
Order a few cartons now, but don't go overboard. If you sign up for the eggcartons.com mailing list they will notify you when they have specials. Sometimes they have 20% off, sometimes they have free shipping. The free shipping offer is great, since shipping normally adds 50% to our cost for the cartons.