Mishimoto radiators are good.
ARC, Blitz, Fluidyne, Gimmick Motorsports, Griffin, Koyo, and PWR are also some popular radiator companies that you might want to do some research on before you buy one.
If you choose to buy a stock replacement radiator, you probably want to avoid using an aftermarket radiator cap. Reason being, the endcaps on the stock radiator are plastic and if you increase the pressure in the system, you increase the boiling point of the fluid and the plastic endcaps have been known to crack. If you want to use an aftermarket radiator cap, you should probably buy an aftermarket radiator that has metal endcaps and is made to hold the extra pressure.
Also, it is recommended that you use a bottle of Subaru's Coolant System Conditioner as well as Subaru OEM coolant, although you could probably get away with using whatever kind of coolant you want as long as it complies with the recommended type in your owner's manual or it's a universal coolant that claims to be able to mix with anything. Also, a cooling system additive such as Redline's Waterwetter, Royal Purple's Purple Ice, etc. is also a good thing to use.
If you haven't already checked it out, this link contains some more great info about your cooling system: Cooling System FAQ: Read if you want coolant information! - NASIOC