How easy is it to find a replacement nib for a PFM? I'm looking at one on ebay with a M nib which is too broad for me.

Keep looking! Find the one with the nib you want. The inlaid nib on a PFM is not changable with out changing out the entire section with the nib in it. The section is color matched to the body so you would have to find someone with a trashed body in the right color that would sell the section. I'd pass and find one with the nib size I wanted.

I was looking for a nib assembly some time ago. Here is the link into the UK, where they have some nib assemblies available. I have no relationship with this seller, just trying to be of help. Most PFM nibs are nibsize F; I believe. Just drop them a mail.
I myself have not acquired one from this seller, I happened to come across a lovely burgundy PFM V with a rare and awesome BB ball nib.

After reading their Q&A here, I don't think they sell PFM parts directly. It seems that they have some NOS PFM parts that they can use as part of servicing a pen you send them for work. This would be in line with what appears to be a common practice among restorers. Parts are very difficult to find. It makes more sense to "sell them" as part of a larger repair job.

Another option in some cases is having the PFM nib reground. Due to the nature of the integrated nib/section assembly, these can't be retipped*, but can be ground. $30-40, and a trip to a nibmeister, could turn that M into something slimmer and more to your liking.

* I believe Nathan Tardiff used to offer retipping services for PFM nibs, but it required that he dismantle and reassemble the unit. I don't know of anyone else who can/will do this.

Regards, greg

There is a post somewhere about the difficulties of dismantling these nibs. As I recall, there are some very small tabs that can break - and if they break, you are done.

So it seems. But when I asked specifically for a black PFM V nib assembly, they made me an offer without demanding I should send in the pen for repair... So may be there is some stock. You have to mention the PFM model and color. Good luck anyway !!