Have a look on the forum
ePER (if it's working). The link to it is in the black band at the top of the page. Click through to the page where you see a box on the left that you can enter your VIN into. Note that the FF
ePER dates from around 2005, so if your car is newer than that, the VIN check won't work.
Once you've put your VIN in, confirm the vehicle details it shows on the following pages are correct. From there on in, any pages you search in
ePER will grey-out the parts that don't apply to your car. Find the page showing the entire gearbox assembly. Make a note of the part number.
Once you've got that, go back on ebay/Gumtree/local scrap yard and search using that part number. Alternatively, stay on
ePER and look up cross-references for other model ranges that use the same 'box. Different gear ratios within the 'box will bring with it different part numbers, so this may take a while to work out. Look at the mounting points in the diagrams. I know that gear pairs from a Stilo 'box can be swapped into a Multipla one (I did that mod myself to raise 5th gear). That's no guarantee the casings will be the same, unfortunately.
Have you tried calling DA Motors? They may still have a Multipla 'box knocking about.