eBay is a good place to get the sensors, I had to replace all four on my 3.5 not to long ago. Usually with the replacing it will help solve the problem. If not try cleaning the sensor with sandpaper like 300-400 that's sometimes work (what the worst that can happen you gotta replace the sensor anyways.)
P1148 Nissan Closed Loop Control Function Bank 1 OBDII Engine Light Trouble Code | Engine-Codes.com
If you do end up changing sensors, look at the tech notes on the website. You gonna have to reset the fuel/air ratio. If you don't then the codes will pop up again. Trust me, after I changed all 4 of my sensors I had p1148 code pop up, them codes stayed in my head, bank 1 sensor 1, bank 2 sensor 1. Hopefully this helps.