Oh, but for a supplier you could probably call SteelArt.
Steel Art or US Sign would probably be your best bet. Fabricating letters that deep would not be very easy as you can't weld them from the inside because, obviously, you aren't going to fit a welding gun inside those deep returns. Instead, you would have to make your welds on the outside & that would mean having a whole bunch of prep work (bondo, sanding, etc.) to make them ready for spraying. I'm sure there are companies fabricating them that way but I wouldn't want to on the team who is tasked to do so.
I think the wire-cut foam is a very viable option - if the vendor can provide a smooth enough hard coat finish so the letters don't look like they have the worst case of goosebumps to be seen in the modern age.