Maniac Music still sells the Sustainiac Stealth Pro. Can get them directly from their website for $229. There's some extra cost options for easier installs for some common models of guitar, too.
Sustainiac appears to be the best commonly available magnetic pickup-style sustainer system, though installs are apparently a bit tricky. (I plan to eventually have in one of my guitars, wish I thought I could adapt to an ebow, as not having to install it would remove the "which guitar" and "what about neck pickups" dilemmas...)
Semi-related aside:
I'd like to see a noise-cancelling hexaphonic sustainer, but so far cost/bulk issues have been prohibitive. I keep wondering if alternative materials might help (silver wire, for higher output with less wire volume? I think there are more efficient audio amplifier designs, too...)
There is a youtube video where someone did a very simple hexaphonic sustainer design, so I'm not sure it's quite so impossible these days... But it lacks the noise tuning, mu-metal shield plate, or any of the other features and options of higher end ones.
More tenuously related:
I'm disgusted at Harman/Samsung killing Digitech R&D, no successors to FreqOut from them...