You need to determine what nib the pen has now. Is it a standard size, #5 or #7? If so you can order replacements of all types from a number of different place. You can spend anything from a couple of bucks to hundreds of bucks on a nib, so find out exactly what your customer wants to spend.
https://www.classicnib.com/bock
https://fpnibs.com/collections/nibs
For everything you ever needed to know, and more about fountain pens, check out
Richard Binder's web site.
Getting in to fountain pen nibs is like falling down a rabbit hole!