As stated, the part is Mavic specific to that type of Mavic rear derailleur. Also, as stated, this derailleur with corresponding shifters won't go beyond 7 speed.
Now if you get a Mavic 841 rear derailleur, that looks more like other modern derailleurs, and their corresponding DT shifters, it will handle 8 speed.

A month or two ago, I corresponded with an member on one of the forums about making extra "plates." He determined that the thickness of the metal and his tools or the lack there of would not be something he wanted to try.

If you can find someone who makes copies of those things, and could get them at a good price, you could make some money because is not uncommon for people to loose that part.