this one too for the o2 sensors, which are very easy to replace. make sure that you dont get the two sensors mixed up.
http://www.automedicsupply.com/catal...ine=L4-1.8L+GT
If you're mechanically inclined then do it yourself. If you do this whole set up yourself, it will save you about 500 to 600 dollars from what you were quoted, and thats with getting good qaulity parts... if you need any information on how to do's, i have the 2000 to 2004 celica manual on disc, if you need me to burn you a copy and send it to you, let me know.