Best deal on AC Delo O2 Sensor?

I replaced my O2 sensor a little over a year ago. Got an Bosch O2 sensor becuause check engine light would come on a little after start up in cold weather. Gas mileage stayed about the same for a while. But the last couple of months its only been at 18 or 19mpg no matter what. I'm used to around 21mpg even in cold weather. Also the check engine light flashed again (actually it was a while ago) and it was a o2 sensor code. I know that Bosch o2 sensors are not reccomended but I didn't have the money at the time and it seemed ok. But I'm convinced that its not doing so well now becuase my gas mileage wasn't this bad with the old O2 sensor ever.

So Where can I find one cheap besides www.gmpartsdirect.com (http://www.gmpartsdirect.com) Their shipping price is pretty high to me. Help would be appreciated.

$34 without shipping? :O that seems pretty expensive to me. Look around a bit more, I've seen them for about 15$. Check ebay. For about $34 I got a NTK sensor, and this was including international shipping.

Went on ebay and found a O2 Sensor for $20. It AC Delco part# 25312184 the right number for the O2 sensor?