Two part answer:
First, for the parts,
https://www.shop4parts.co.uk are a Fiat parts specialist and I highly recommend them
But the second part: the rear discs are nearly on the far side of impossible to remove... you need to have lots of time, maybe some extra tools and ideally a lift. I’ll search for, find, and then link here in an edit the post that helpfully explained how to make the seemingly impossible possible
[edit] Read the thread here:
https://www.fiatforum.com/panda-iii/437398-4x4-rear-brake-discs-3.html?p=4377889 - this link goes to post number 42 that (along with the one that follows it) explains how to get to removing the disc (the challenge is gaining access to remove the calliper bolts). you might read back through the whole thread to see what you are up against, maybe starting at
post #13 which summarises those that precede it