Cheapest place for most everything.
The front one, commonly referred to as an O2 sensor, is an air/fuel sensor that tells the ECU if the mixture is correct. If not, the ECU will adjust as needed. When this one goes bad, the engine starts running poorly. If this one goes bad, you have to replace it. It typically goes bad mainly due to contamination and that is commonly oil consumption and a lot of soot build up. Heat can damage them, but it pretty much requires a torch directly to it for a while. I mean they are designed to cope happily with exhaust gases that can get north of 1000F. Unless the tech melted wires, they're hard to damage just from exhaust work elsewhere.
The rear one, also commonly called an O2 sensor, is a cat efficiency sensor. It just makes sure the cat works. If not, the ECU will throw a code for cat efficiency. The rear one does absolutely nothing for how the car runs. It can even be removed and nothing will happen outside of the check engine light staying on. With a lot of the aftermarket headers, this sensor triggers quite easily. There are eliminators and things you can do to help it not trigger, but really it doesn't matter unless the check engine light bugs you. I'm not sure how much you did with your exhaust, so I don't know if this is avoidable with your setup. If you are running an aftermarket header and/or high flow cats, you may just have to expect this to always be there. You might look at an eliminator.