Most recently, for a discontinued Audio Technica set that did not match any of the standard AT pads, I got replacements from Wicked Cushions.
I got the impression that they've had phone parts in stock for a very long time, but of course that's relative to the average phone age and I don't know how many years you'd be expecting. In this case, though, if you already know what you'll need ahead of time... that's a cheap thing to just keep in a drawer. Can't go out of stock if you already have them and thereby double the other parts' longevity.
I originally thought this was about in-ear buds (because of the somewhat misleading name), for which they'd have been just a few months too late for me (I bought some Sony MX4s earlier this year), but full over-ear headphones? I might be interested in that.