I did a bit of reading about aftermarket radiator caps a while back since I was interested in adding some under-hood bling, but decided against it due to potentially overpressurizing the system. The stock radiator has plastic end tanks which are likely to crack if the pressure is increased much beyond stock specs. The system already experiences some fairly significant heat/pressure cycles as the car is driven and you should really only consider an aftermarket radiator cap if the stock radiator, hoses, and thermostat are also upgraded.
It appears that SubaruGenuineParts.com has a stock radiator cap in the OEM catalog under: Parts - Cooling - Radiator & Components - Cap for $13.85 (probably plus shipping)
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)