I use Exterior Roof/Walls plywood 3/4". Very good quality, doesn't require a ton of sanding, holds a lot of weight, and is SOOO much lighter than MDF or Melamine. I believe it is grade A or B at least. $21 a sheet at Lowes.
I'm very surprised you didn't find melamine at Lowes, tho. They have it in two places: the shelving/organization section as pre-cut shelves, and the lumber section with the presized boards. 12 and 16" widths. I got some for a rack, but changed my mind when I had to carry it in from the cab. That stuff weighs a ton! Now, I plan to use it to make a hatchling rack...with casters.