EasyParts has rear shocks for Mp3 250 and 400 in stock; just the shipping will be a little pricey. If you can find someone to install it for you then you should be all set.
I checked the various permuations of Mp3 250ie and each parts diagram path lead to the same Rear Shock part and URL.
https://www.easyparts.nl/Rear-Shock-Absorber-(1-pc)-p33525943
Basic Mp3 250 series start page.
https://www.easyparts.nl/models/Piaggio-MP3-250-m4062
From there drill down into the parts till you find what your looking for. Make SURE you know exactly which variant of the Mp3 you have so you know the parts will fit. The Rear Shocks should be generic because every parts-path I took for the Mp3 250 NAFTA, EMEA, and sub-variant all lead to the same rear-shock part number. Which makes sense as they're all the same Piaggio 250 engine.
For the Mp3 400, it appears to be a different part based on the URL being differant than that for the 250.
https://www.easyparts.nl/models/Piaggio-MP3-400-m2468 Which leads to
https://www.easyparts.nl/Rear-Shockabsorber-p33559440
The official Parts Piaggio Page lists the rear-shock as part #58511R after going through the Mp3 250ie series part diagrams
https://www.parts-piaggio.com/piaggio-scooters/assignment_spare_parts/58511R
Once again, regardless of which parts-diagram path for the Mp3 250ie I take, it all goes back to part 58511R.
https://www.parts-piaggio.com/piaggio-scooters/250-PIAGGIO-SCOOTER/MP3/2008
The Mp3 400 series rear-shocks have a different part number and are implied to fit only 400 and 500cc bikes.