Uline is a good choice, for sure. Lots of options. Sometimes after including the cost of shipping the boxes to you the costs can get pretty high, though.
I have had a lot of luck acquiring boxes from frame shops (that would otherwise be recycled)- boxes that had transported 32x40 or 40x60 inch mat board, foam core, etc. These boxes can be wonderful for shipping.
Alternatively, a custom crate can be easily built from 1/4 inch plywood and lengths of 1x2 doug fir or pine. Many home improvement shops will even cut the pieces to size for you and you assemble with screws. It's a strong, fairly lightweight, and professional looking option... Hope this helps.