I'm looking into them also. Rockauto.com had some Denso ones for about $40 a piece. It says Denso is the OE replacement on their site so I figured I might just go with those. They have ones that are a little cheaper and more expensive also. For that price, I would just fix it. I'm doing mine soon because gas mileage is suffering and I've done everything for a tune-up except that. I'm pretty sure our vehicles only have one (mine is 94). Its a rod-looking piece connected to wires that comes straight up from the top of the exhaust near the front of the cat. Service manual says it will have two for California emissions, with another one after the cat.