Shower Head Part or Replacement

My brother is trying to troubleshoot his broken shower head. Inside the shower head is this part which has calcified and broken apart (pics. 1 and 2) He went off the the hardware store to see if he could just buy the replacement insert and not the entire shower head, but they told him no.

1. Is there a way to get a piece like this (the name of which I don't know), or is it always necessary to buy a new shower head?
2. If the original shower head is discontinued (Grohe Relexa-- pic. 3), how can we figure out which newer Grohe shower head would also work?

I did find one on Ebay, but does it make any sense to buy a NIB older shower head? The advantage is that it would match his other bathroom.

Front side

Back side

Stationary shower head

TIA