Try this page for parts sourcing info:
viewtopic.php?f=6&t=34980Exhaust sensors are usually made by Denso, the oem Subaru Denso sensors can be very expensive.
Front a/f sensor
The more affordable sensor will be the Denso oem equivalent from eg Cooldrive or global efi etc, same as oem but in a denso box.
Members have also had luck with the goss oem equivalent part number.
But follow the testing procedures to potentially not ruin a brand new sensor.
Or source a known good used sensor and see if it fixes the issue, but risk ruining it eg if the originating fault affects the sensor, eg incorrect afr due to inlet tract leaks etc causing rich/lean conditions.