if you're still on standard wrx shocks, then you can replace them with KYB shocks (who make the Subaru ones anyway, i think).
Saj's ones will be used shocks, whereas you can get the kyb's new for around £80 each. Replace both rears, reuse your springs and top mounts.
Have a look at Camskill's website , i got mine from there.
http://www.camskill.co.uk/products.php?plid=m32b0s0p0