Appears to be an XF-115C for sale on eBay.co.uk. Not it’s not me selling it. You may need to modify the mounting PCB for an 817 as it claims to be for another Yaesu radio. Not difficult if you know which end of the soldering iron gets hot!
All my “filter accepting” radios have CW filters fitted so I have no horse in this race.
Cheap at £90 
My 817 narrow CW filter is also not for sale - probably a collector’s item one day like my 21-year-old 817.
I remember seeing a fellow first-year E&EE apprentice at Marconi, Chelmsford (in 1969) accidentally grab the hot end of a soldering iron. As he dropped it the skin on his palm came away in long sticky strands. Decades of breathing in the fumes when soldering probably explains my current mental state.
You could buy a QMX kit for not much more, and it would probably knock spots off an 817 on CW filtering…
I’m looking forward to mine arriving soon!
Yes. But no!
I’ve always found my Yaesu YF-122CN CW narrow (mechanical) filter rather good and much better (being an I.F. filter) at coping with strong adjacent-frequency signals than, say, the DSP-based A.F. one on my FT857.
Apparently [according to K6XX] a mechanical filter had poorer strong signal selectivity but a cleaner in-passband tone than an equivalent crystal filter but its ‘skirts’ are not as good [No jokes please].
It’s probably invidious to compare the performance of the 1990s-designed FT817 or its filters with modern software-defined radios (like my KX2). In any case, I assume any buyer of the filter wants to continue using his FT817 [rather than sell and replace] which still serves as a good all-rounder all-bands/most-modes QRP rig for SOTA.
Exactly Andy. There are people who can listen to many CW signals in an SSB bandwidth and mentally tune out the interference from the signals they don’t want. I am not one of those people. So I need a narrow filter for CW. There will be many with FT-817 owners who’d find such a filter useful as it massively improves the radio’s usability.
I can agree with all comments above! I have an 817 with CW filter and it does make operating much less stressful on a busy band. The radio also covers all the commonly used bands, using CW /SSB / AM / FM and is good as a tuneable IF for microwaves. I won’t be getting rid of mine, or its CW filter.
However, if I didn’t have the filter, I’m not sure that I’d be chasing after one.
A QMX (for example) would add more to my SOTA armoury for about the same outlay - smaller, lighter, less current draw, modern design including better filtering.
Whether I still think that once I have it to try is an open question 
When I first got my beloved KX2 I used to turn the (continually-variable) narrow filter down to about 150Hz or less. However, following advice some years ago from sadly-missed Guru EA2IF - that I was not hearing well or missing chasers calling out of that passband - I broaden it now to about 300Hz [unless there is strong QRM ‘next door’]. As he said, one has to train one’s ‘ears’ to cope with more simultaneous signals and a bit more background noise.
But there’s no doubt in my mind that using a wide filter (e.g. SSB) would be throwing away a better S/N ratio and the ability to hear those weak-signal chasers.
You’re right Andy, about being cheap at £90. I paid £120 for a 300hz filter for my 817 from the same source about 2 years ago. It does make a huge difference though!
Even the cheap SOTABEAMS audio cw filter makes a world of difference in the “barn-door” 817.