Locating a Sterling replacement grill, any suggestions?

No and they must not be available or you would not be asking. I would cut out a piece of wood a bit thinner and glass it in there. Sounds like a big job but it is not if you can do glass.
Looks like you are from close to the water but the high rent district. If you get away from those tall buildings and are laid over close to the water look for a boat glass guy. Should be able to get that fixed up for 200 bucks or less. Paint it chrome silver. No it will not look perfect but you won't look like a crack lizard missing teeth.

 

Sterling AT 9500 Series Grilles | 4 State Trucks

 

Are those trucks in general difficult to get parts for now?

 

They were a PITA to get parts for a decade ago lol.

 

That sucks. I like the older Ford trucks like the LTL models. But then I found out that certain parts are hard to get and there are some parts that are incredibly outrageously expensive.

It's a shame because I do like them but when I found that out that was the end of that.

 

Certain pieces for that style grille you CAN still get. My father has that style, it’s pricey. Jones Performance used to sell everything needed and it was upwards of 5000$. Try getting with them. I have a spare “old style” from my parts truck. Sadly they Mount slightly different. I wrapped mine in stainless steel mesh. Probably 300$ in my grille.