Hi, my mom’s car is a 2015 Kia Optima SX 2 liter turbo with 100,000 KM. 

The check engine light came on 1 week ago with an evap leak (fixed by changing gas cap) and another code (P0138) for O2 sensor circuit voltage high on b1s2. I erased the code to see if it would come back as everything was almost perfect in live-data and it rode fine but it came back yesterday, but this time in live-data, I had a short term fuel trim of -10% and you can feel it run a bit weird. 

I went to the dealership to replace the sensor but they are crazy expensive, 450$CA for a Kia branded sensor and 517$CA for a hyundai branded sensor. Even the guy at the dealership straight up told me that this is BS and that it doesn’t make any sense, and recommended me to go buy it somewhere else. At the local auto parts store, they told me that they can’t get the sensor 2, since Kia doesn’t share it and there is no aftermarket for it, they can only get the sensor 1. 

What should I do? Try a cheap sensor online? Buy it from the dealer even at that price or try my luck at a junkyard (im in the northern cercle in Canada so my chance of finding is basically zero…) 

Also, along the O2 sensor code, it gave me a P0299 for Turbocharger underboost, could it be caused by the O2 sensor ruining the fuel trim? 

Thanks have a good day.