Rock Auto has Denso and NTK o2 sensors for reasonable prices. The Denso sensors are made here in the USA, instead of Japan, but I would not think that would create a problem.
You have 4 sensors, Typically the ones in front are the culprits. If you want to make sure, have a shop(or club member with a reader) hook up a reader and read the sensors out puts first. Check to make sure the cats are heating up as well.