Can't send you the link, but less than 50 miles from you is a Lumber co, that has Cypress Plywood
Cajun Cypress and Hardwoods, 14251 Brown Rd, Baker, LA 70714
Otherwise use a Marine grade plywood, or
HDO or MDO plywood, this is the Sign material your State Hwy dept. uses.
As for treated wood, they don't make the one that had Arsenic in the formula, and is EPA approved for garden raised beds.
What my students use is a 12 ga, 1.5" sq tubing with 10mm PVC Coroplast signage material. By the way after the frames are built and welded we take them to the Galvanized plating plant and they Hot dip them for us.
By using Coroplast no liner is needed.