Hello,
The AD633 is a rather expensive device at AUS$40 but I can supply them if required. If someone has done a large volume buy (and I mean at least 1000 pieces) then the price drops to a respectable AUS$9.
A replacement will depend on, amongst other things, if you have an existing pcb or are building the circuit from scratch. If you already have a pcb then the replacement will need to be pin-compatible. I don't know of any direct replacements so substitutes will probably need some of the component values around the chip to be modified.
Best Regards
(Mr) Laurie Biddulph
Phone: +61 (0)2 4340 0938