Justin Chancellor doesn't even use the 5th up / 1 octave up harmony all the time. He uses other modes, too (particularly the standard 1-octave-up whammy mode).
But to the OP: there are a lot of good threads that have covered this recently. Clearly, you've already taken a stab at searching if you know as much as you do already... it wouldn't be that hard to do just a bit more research, given that this comes up ALL the time.
IMO, the closest you'll get to the quality of results from the Bass Whammy would be with a Whammy IV. You won't have 5th Up / Oct Up, but it'll cover most other stuff, and it sounds cool regardless.
The PS-3 and XP-100 let you get the "modes" unique to the Bass Whammy, but they don't track or sound the same way. The PS-3 is in a class of its own when it comes to tone...
