Something like this?
http://www.homedepot.com/buy/lithonia-lighting-4-ft-replacement-diffuser-275605.html

Thanks for trying, but I can only reply, maybe. Correct me if you know different, but yours looks like it slides into a different type of troffer than I have. Mine has some tabs that are punched out of the troffer that hold a solid, lens or diffuser with boxed corners (5 sides).

For the lens, visualize a shallow ~2 x ~4 tray with sides 1 or two inches deep. There are slots in the sides that mate to the tabs on the metal, lay-in troffer.

Yes, the replacement diffuser is probably not a direct match.

If you need to replace the lens, the cheapest approach is to get an acrylic sheet (or something similar) and shape it to size.

Otherwise, you'd have to consider replacing the entire fixture.

Hmm, that is bad news. There is really no way to support an acrylic sheet unless I lift up the whole troffer and slide the sheet in on top of the ceiling grid. I have seen replacement diffusers, as opposed to lenses, that are actually designed to be installed that way along with higher efficiency reflectors that insert into the top of the troffer.

I suppose a trip to a dedicated lighting supply house my be my best bet. They might be able to supply me with some aftermarket hinged insert that can save me some time though it might be a little cheaper to replace the whole thing. The troffer there really does look like a high quality model. There are wing nuts on the ballast cover and other parts are relatively robust. The former owner/electrician used good stuff when he could salvage it.