It sounds like you may have already rooted your phone? If so, I am not sure how we would get you the official updates as they are sent automatically. You may just want to peruse XDA and see if you can find a trustworthy one.
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You can find stock ROMs on Sammobile.com. I had to use them to go back to my stock ROM after a coworker buggered up my phone when trying to root it.
Just sign up for the free account and let it take an hour or two to download. Instructions on how to install the ROM / Firmware are on the right side of the page.