I think your m/c is a PS16 isn't it? If that's the case then I've never managed to find a rebuild kit for that particular size; PS11 - yes, PS13 - yes, PS15 even - fine. But PS16 kits seem conspicuous by their absence.
This thread from the Paso forum contains some useful info. and says that the rubber boot from the PS15 can be used on the PS16:
http://forums.ducatipaso.org/viewtopic.php?f=1&t=8289
It's a bit extravagant but, if you're desperate for the dust boot and can't source it separately, you could use one from a PS15 repair kit. I think this is the right one:
https://www.ebay.co.uk/itm/MOTO-GUZZ...0/264738324285
Alternatively, if you can find a KTM with a similar PS15 or 16 reservoir, you might be able to buy just the boot from them as KTM don't seem to have a problem selling Brembo service parts.