As you may have now realised finding a space saver for a Subaru is not easy from the recognised tyre/wheel websites.
Subaru will sell you one as a whole kit close to 400 notes. This includes a new floor mat and other parts for your boot as the new spare will raise then floor of your boot. The reason for all the hassle, cost and buggerance factor is because we have bought a Subaru!!
If your space saver rolling circumference is different (usually smaller) than the wheel/tyre on the other side of the car then (because of the AWD) your differerential will be put under a lot more stress and occasionally by the time you have driven to the garage then your diff. may be knackered.
The up shot is, if you buy a cheap space saver and it is not very close to exact size of your circumference of the other wheel then be warned.....
Maybe you just need to get a full size wheel kit from a dealer(£400ish) and lose some boot space, rely on Green Flag, use the repair kit or trust in God.