My wife manufactures and ships products in lots of different size boxes for her business, and I've been down this rabbit hole. Truly custom boxes will cost you an arm and a leg for a one-off project. There are thousands of vendors out there, and they've all got lots of options, so you might not need to go the custom route. Do you have a picture of what you're after? Sometimes you can match up with something on Uline, or another commercial supply house, and get a good deal.