For anyone interested, just found this shop in ebay is selling WMO's with the original packaging (OEM) which is pretty rare nowadays.

http://www.ebay.com/itm/Microsoft-Optical-Whe...5aead2db2b

Since I don't trust the Chinese plus it would take ages to arrive, and in case bst mouse fails to be a proper WMO replacement you may want to check these out before they are gone.