Where are these O2 sensors

I keep reading replies and threads that are mentioning Bosch O2 sensors for like $30. I’ve only been able to find Bosch oem for like $90 and a universal for $70. Maybe this might have something to do with 90-91 having non-heated sensors and 92-93s having heated ones? BTW I have a 93 ls. Point being where are yall getting these cheap Boschs?