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I have a 4 point Sabelt harness - mounts to front and rear seat belt mounting points - you replace original bolts with eyebolts so harness can be clipped in and out.
Usual caveat applies - ie beware 'cos if you have these & no cage then if you roll, it could possibly do you severe head/spinal injury due to the inability of your body to move sideways when the roof compresses
Otherwise any rally spares type shop with prices from £30 - £100s
Other standard issue is that the standard issue Subaru seats aren't designed to deal with the load generated by harness straps over your shoulders. Therefore, to use a harness, its recommend that you install fixed back racing seats.
I don't remember the exact details, I just know I took my harness out after reading a lengthy thread on it.