You might want to price 3/4" marine plywood. You can cut down one sheet and
probably replace 4 to 5 of the shelves.
Home Depot's and Lowes' websites don't seem to show marine plywood, or any
plywood in 3/4" thicknesses, but you could call around to local lumber
yards. You don't have to go specifically with marine plywood...
TTFN
Andy-roo