Some galant GSR's came with it - very cool system that changes damping for cornering, acceleration and braking, and adjusts ride height based on speed, backing up and opening the door, but its major failing is that the hydraulic pump can only stand up to about 15 minutes of "spirited" driving before it overheats, throws a warning light on the dash and has to be allowed to cool off.
Parts are probably pretty hard to find outside of used parts in a junkyard (where they turn up every so often), and retrofitting it to a non-GSR would be extremely difficult, due to the large number of electronic sensors and hydraulic lines running everywhere in the car. There's a big section on it in the Galant factory service manual.
A more simple version of the ECS (as opposed to Active-ECS) that only adjusts damping is what is used by the 3000GT VR-4 - the hydraulic system is eliminated, and only the electronics and stepper motors on the dampers are used in that case.