If you don't get any better offers, I have a generic 56k external that does work.
It would come with modem and PSU. I don't have a spare serial cable.
It does say on the bottom of it that is is a Rockwell. I don't know if it's a V90 or not.
It does work well. I'm not using it now as I replaced it with a Aopen modem. I think that I had it originally just set up as a Generic modem. (in Windows, not sure how Linux works in that respect)
If interested, make an offer.
I would be shipping from Canada.