The place I get steel is a "backwoods" steelyard owned by Leonard Looper and mainly run by his grandson.
They cut the length you want on the spot with an oxy torch, so I usually ask for an inch longer than required in order to cut off the slag. They pretty much only have hot-rolled steel, but a good selection of sizes including square tube. In this case I'd figure out the total length of square tube needed for the project and get single pieces that can be cut apart with a chop saw, as long as it fits in the truck.
For the step, I'd get 3" wide rectangular tube. Measure the space between the tires and allow 6" clearance on each end. Allow more if you plan to go up in tire size in the future.
For the supports use 2" square thick wall tube. Measure from the frame to the side of the body and add a couple of inches, then multiply by 4.
For the clamping plates, measure the vertical size of the frame and add 2" for the height. I'd make them 4" wide. You need 8 of these. I mentioned 1/2" plate, but 3/8 is thick enough.
Then measure the thickness of the frame and add 1.5" to get the length of the bolts you'll use. 1/2" grade 8 x 16, plus nuts.
If you get me the measurements I'll call Loopers and get you a quote on the steel. The bolts and nuts you can get at Tractor Supply.