They're easy to get to (the first two, don't know about the third -- I only have two -- but suspect all are easy to get to, especially with the car on ramps). I think on yours, there will be one on each "Y" pipe, and another just before the converter where the Y goes into a single. Just a guess.
The least expesive O2 sensor I found was here:
http://oxygensensors.com/
They have them available with the connector, or without. Depends on if you wanna go to the trouble of cutting off the connector and doing a splice, or not.