Is yours an import ? I doubt if will have the sls units as standard, have a look at the rear shock legs if there's a remote reservoir attached to the leg then it's an sls unit, if not it's bog standard,
So you can fit normal shocks with springs, kyb do shocks and you can get custom made springs from various places, Pedders also do a lowering kit and it drops the car by about 30mm
You can also use impreza shocks/springs however it will sit low at the back as ideally you'd need to pit a spacer inbetween the topmount and car to take into account the different spring rates between the models, finally if you do go down the impreza route, you'd need complete units from a bugeye wagon as the tops are different from classic impreza, assuming you don't re use yours