Does anyone know of a source for restoring the original (but not the "D" type) of 240z wheel covers...?

Can you shows us some current pictures of your wheels covers?  You would need to find a chrome plater that can and will plate plastic.  There are some other method to get chrome on to plastic that you can research.  Once plated, the wheels covers would need to be masked and painted.  Nice sets of non-D type wheel covers come up for sale here and there and are usually pretty reasonable.  Given the labor involved in restoring a set, may be cheaper to get a nice used set.

I found a set of four on Ebay. Looks like 2 are almost perfect, 1 is very, very nice...but the fourth one will not go on my car in the shape that it is in. So I am now looking for one or even two that are immaculate. And on that note, I have at least 4 to sell that need restoration.

Yeah, that is usually the case.  You end up with several sets to make one really good set.  If you go down the restoration path, the chrome plating of the plastic is the biggest challenge.  It is unfortunate that once they are painted, very little of the chrome is revealed but there is not substitute for the trim as it would look wrong if it was just silver.  Check out the place below, they may be worth calling.

https://www.muellercorp.com/custom-chrome-plastics-and-restoration/