I know that evilcalvin installed a CAI he bought from eBay back in March 2011 for $80 and was happy with it. He drives over 25k a year so by now he's had some long-term experience with it. He lives in CA like me so we have to worry about smog and diagnostic codes. Last I heard he does not have any problems. Link to his install:
This CAI is from Zomk Speed. Other people have tried other aftermarket CAIs and some have experienced various problems. I mention this one because of cheap price and no problems. I don't own an aftermarket CAI. Your experience may vary.
I just noticed in your post you asked for a TRD intake.
These are not TRD, but I thought I'd mention them. I found eBay listings for CAI for Scion xB from Zomk Speed but not sure if this is the same one as on evilcalvin's car. Available in polished, red, black, or blue. Search in eBay for
spyder scion xb cold air intake
I am not affiliated with the seller, I just like a good deal. Let us know how it all turns out, and take pics!