Both of my catalytic converters are starting to break apart in the inside and rattle. All the exhaust shops I have been to are giving me a price around $800 a piece to replace without labor. Since it is a G35x awd, they have said I have to use the stock cats from infiniti to replace only and there is no aftermarket cats that will work. Does anybody know any aftermarket cats that will work to replace the stock? I have looked online and since its the awd model I am not having any luck. I would think an exhaust shop could make a 370z cat or something along those lines work, please let me know what you guys think. Much appreciated.

Doing a quick lookup I don't really see a different part number between AWD and non-AWD catalytic converters so you should be able to use aftermarket. You can always drop by your local Infiniti parts department and have them compare the parts in FAST

You can purchase after-market converters for almost half the price. You need converters for the HR/VHR motor. We carry them on our website.
Just type in the search bar, HFC and it will return the ones we carry.
Tunerz Store

If they are starting to break up, get them replaced immediately. You don't want those pieces sucked back into the head, it will scar the cylinder walls and cause oil consumption issues.

awesome thanks guys.