Not all narrowband O2 sensors are the same technology, and their response characteristics do vary between technology types. Our ECU is rather finicky about that.
A new O2 sensor from lotusgarage.com is $177 (not including shipping).
$147 (before shipping) at mylotusgarage.com
$145 at blackwatchracing.com
Monkeywrenchracing sells a universal O2 that they claim to have the same response characteristics as the OEM one for $59. You just have to splice in your stock O2 sensor's plug.
Lotus Sensor - Oxygen (O2) - Univ Replacement - OSM-O2BOSCU van Lotus - Lotus Sport - Monkeywrench
If anyone has tried this sensor and can report on if it worked without any problems, please do so.
People have reported using other O2 sensors at this forum, some with success, some not.
Both upstream and downstream O2 sensors are the same.