First of all - the upstream sensors are the ones to stress about, they are CRITICAL to how the car runs! - the downstream (expensive) O2 sensors are strictly there for emissions monitoring, to let you know how the cats are doing their job!
So if you have required emissions testing in your area, (or are just concerned about the environment) then the downstream sensors are important, if not, then they are just an annoying Check Engine Light
By knowing the original Lexus OEM manufacturer (NipponDenso) and the correct part number, I was able to purchase BOTH of my upstream O2 sensors for about $50 each at amazon, which was less that 1/3 of what the dealer wants for the EXACT SAME PART!
The installation of O2 sensors is a fairly easy repair - harder than changing an air filter, but much easier than, say, a timing belt - so you can DIY and save even more!
PS - your ECM trouble code that is indicating a bad O2 sensor is capable of being decoded and will let you know if the fault code refers to an upstream or downstream sensor!