It is hard to tell grid size from the picture. Unless quite small, these would not meet building codes in many areas. Presence of horizontal components would allow small children to climb like a ladder and go over the top. Many areas prohibit horizontal balusters for this reason. Thin Iron or aluminum balusters are common--if you eliminate the horizontal elements. Try looking at fencing suppliers rather than deck railing since they are not bound by the same code limitations if you still want to go forward with this. There are quite a variety of fencing styles possible.

I would guess the grids are roughly 2x4" in size. I see your point with the climbing hazard for children. But since this was constructed on Federal Property in a National Park it seems as though it would be up to code.

That would seem to be logical, but we are dealing with the government! ;) Building codes are a local matter and the federal government is generally not bound by local rules. Some areas will allow, some will not. I mention this as a reason they will be hard to find as "Deck railing" As long as you are comfortable with risk and either local building inspector allows or you are willing to build without inspector approval, you will most likely find this as "fencing material". Porch (near ground level) and fences are not subject to as much regulation. Manufacturers are at less risk of getting sued if someone should fall if you use their product for a use other than the intended.

For example, blackdiamondfencing.com sells 4"x4" grid deer fencing. You may need to look at a number of sites to decide what is closest to what you are looking for, but fencing sites are likely your best bet short of contacting the national park people.

A variety of fabricators will make a custom sized iron mesh and coat it with PVC or whatever, but these custom products are probably not practical at your volume of purchase. I mention this because the park service can buy in large enough quantity for custom fabrication.