Check out Home Depot or Lowe's. They have cardboard boxes. I can't say if they have them to fit your dimensions. You might have to get creative and buy 2 boxes and incorporate them, tape them together to make one shipper.
Understand that you're looking for a custom box. If they don't exist on a normal basis, then it's going to cost. And what UPS does might not be any different than buying two boxes at Home Depot and taping them together yourself.
Christmas tree boxes are a great idea, because you can buy 'body bags' for storing Christmas trees. Bed Bath and Beyond carry them. Bronner's has them. A lot of these home decor mail order companies carry them. I'm sure hardware stores would too. Anyone selling Christmas trees would have the boxes or body bags