If you haven't already seen:

Oakos Automotive - Subaru Impreza WRX Performance Parts : Weights of Various WRX Parts

You can try a aftermarket U-brace like this one or the gt-spec:

RenickMotorSports.com

Here is a place to start for wheel weights:

WHEELWEIGHTS.NET

These are light 13.7pds. Kosei K1 TS

Wheels for 2006 Subaru Impreza WRX Sedan