My floor register boxes are rusting out, but none of the big box hardware stores have a replacement. Where do I find a 4x10 x 14" deep boot?

Unfortunately, it appears to me that the ductwork was not embedded in the concrete, as you would think it would be by code. Since a lot of dirt & sand has infiltrated the rusted out boxes, & there is no concrete below it.

Also, there appears to be a plenum that the tabs just go through the box which is just a plain box, no extensions, angles or anything else. So it would just be a plain box with a hole cut in it at the side bottom.