Depending on how many boxes you think you will need - there is always the option to get custom made ones and import them (e.g. from China). But this is only worth the effort if you order large quantities. Maybe even a local cardboard/box retailer/producer would be willing to produce fitting ones for you. But this also would mean larger quantities.
On the other hand, I don't know if this might not impact the price too strongly. Cellophane bags are very cheap, art cookies are very time consuming in the making. You will already probably work for a very, very low hourly wage. I doubt that you will get a considerably better price if you sell them in what kind of boxes ever. So in the end you will only further decrease your wage, I fear.
But maybe I am wrong, and you are so fast that you can even sell your extraordinarily beautiful cookies at a price that people will still be willing to pay - even with additional boxes 