personally, if i were you, because as you said Sony don't sell this item, so, i would take that personally, & buy a Logitech RX1000 (thats what i have, & if i remember correctly, it comes with a PS/2 adaptor, if you need / want to use it)
BTW: because Sony have basically said bo11cks to you by not providing replacments, i'd say bo11ock right back to them & buy the Logitech
the RX1000 is a 1000 DPI Laser mouse that works on nearly any surface (except water) & i'm more than happy with it & i use Linux & it works great