Like several others on this forum, I got a cheap K2015 THD off e-bay (though after all my replacement parts etc it is probably not that cheap anymore!)
It is probably around 15 to 20 years old so the display was rather dim. I checked the voltage and it was 36.4V which was close enough to the 37V that is the nominal value so I knew it was the VFD display rather than any caps (I'd replaced all the electrolytic ones anyway).
Without a great deal of hope I approached Keithley (now Tektronix) about getting a replacement display board - no luck with this but I also asked them about the display itself (part number DD-52) and they were happy to sell me this for £53 + VAT.
Unfortunately to fit this you need to desolder the old part (34 pins) and lever it off the board (it is stuck down with double sided tape).
Having just acquired a Denon desoldering gun off ebay (thanks to dunkemhigh for putting me onto this) the desoldering went fairly smoothly though I had to resolder and repeat a few times on a few recalcitrant pins.
The problem came with the sticky tape and a few odd threads of solder perhaps which required me to gently lever off the old display - one of the 34 tracks lifted from the board!
I carried on regardless, fitted the new display and attempted to solder the wayward track to the side of the display pin.
I put the K2015 back together and the display worked except for the left hand center segments
I took it all apart again, scraped the pin and track with a razor under a magnifying glass and had a second soldering attempt, this time the solder seemed to take better. Put it back together and now the display is ok
I meant to record this saga in photos but forgot so here are a couple of photos of the old display after removing and a photograph showing the new display nice and bright (the same as my much newer K2000).
By the way, the display part DD-52 seems to be common to all Keithley's models (K2000, K2015, K2010 I think if you look at the service manuals).