The best place for it would probably be where the second cat is. A high-flow cat should fit fine, as long as you get it with the proper size, but you may need an exhaust shop to weld a flange on so it bolts to the flex.
In California, IIRC, you can replace your catalytic converter if it was damaged beyond repair (and not "oh no! someone bashed my cat in with a tire iron in the parking lot of this performance shop!" damage). It must be replaced with a certain type of aftermarket cat, which don't flow as well as high-flows.
Edit: Those of us in the wondrous flyover states can replace cats with anything we feel like. Some states will even allow straight-pipes, as long as your car passes hydrocarbon testing.