being a GB Micro owner myself and an Ebay-er I would find someone with an auction that was sellling a unit with multiple face places and offer to buy one seperately from that auction. ( I did not mean outside of ebay)
They can relist thier item[less one screen front] for pennies in most cases and make something to boot or even ask they ultimate buyer of THIER auction if THEY are willing to part with one and introduce you.... I have seen them listed (not at this moment) as accessories, they list a package with all the trimmings and includes a few faceplates. So in conclusion, sometimes it's easier to find when searching under another item, same system.ie don't search faceplate, search 'GBM front cover' or even 'Game boy Micro Front Cover' Although they look the same the search engine doesnot always reveal it the same way.
I Just sold one weeks ago and had an extra myself. But that's history.
Hope this helps.