The space saver's in the V7 onwards STI will clear the brembo's. They have a bigger hump in the rim. But it wouldn't be safe if the other wheels are 18's.
I think getting one from a new STi would be the go. If it would fit in the well of the boot, diameter may be to big. I doubt a full size inflated 18 would fit.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i