Original nibs as spare parts are only available via Montblanc itself; used nibs can practically only be found via eBay, e.g. by acquiring a further 146 donor piece. The nib swap is possible on the Meisterstück; the weak point, however, are the lamellae which can easily be bent when handled; therefore extreme caution is warranted. The procedure does not differ significantly between Montblanc, Montegrappa or Faber-Castell. Anyone with little experience should first practise the procedure on cheaper pens or commission an expert.
Related pages on fountainpen.de:
• Montblanc Meisterstück 146 (Le Grand): https://www.fountainpen.de/c-montblanc-meisterstueck-146-en.htm