There was a guy here the other day that went to Lowes and got the Chrome plated wire shelving they sell in 12", 16" and 24" depth and up to 12ft long, you can then cut it down to whatever size you need for shelving. It seemed to work well for him, and was an easy to find solution.
You can find expanded metal at most farm supply stores in 4x4 panel sizes.
I built mine from wooden frames with metal grill toppers sitting on top of the frames, but my smoke chamber is 22" wide and 16" deep, a bit smaller than what you have planned. I just wrapped the wooden frame with foil to keep the fluids from soaking in. Easy to change out and replace as necessary.
A lot depends on your temps you plan on cooking at. Mine max out around 250 degrees so I have more flexibility than some of the guys that crank it up to over 300 degrees.
You can also locate some old oven racks from a salvage yard.
There are tons of options, some guys even don't use racks, and get stockingettes and just hang all of their food in the smoker, you can even make a hanging basket style container with a grill top wok (the one with holes in it) some stainless chain and a hook on a rod at the top of the chamber.
