We are going to assume the defective oxygen sensor(s) indicated by your Toyota's computer is the upstream oxygen sensor. This the O2 sensor located before the catalytic converter. You can find Toyota Tacoma oxygen sensors (upstream) online for around $125.00 a piece. We'd recommend replacing both Bank 1 and Bank 2 (left and right) at the same time, since they probably have approximately the same wear, unless you've change one in the past. Bank 1 is the side on which the number 1 cylinder is located.
Replacing the oxygen sensors are pretty simple. They are connected by two bolts, each. If you do the repair with the battery disconnected, your Toyota Tacoma's check engine light should turn off after the job is done and the battery reconnected; but you'll need to drive your truck for 100-200 miles in order to complete the "emission monitors". Only then will it pass the smog check, assuming no other trouble codes exist and your Toyota passes the emissions portion of the smog test.
If you are not mechanically inclined we'd recommend you have a smog check repair station conduct this repair, especially since its for the purpose of passing the smog test. Without diagnosis, a smog check station will probably charge around $450.00 for the complete job; maybe a bit more if it includes diagnosis and a guarantee the check engine light does not come back on. In other words they diagnose the truck first, make sure the only problem is with the oxygen sensor, and then make the repairs.
