Which Stock Firmware for Galaxy S3 I'm in a situation where the Galaxy S3 I have is not connecting to Wifi (bug from Android 4.1). I would like to flash a new firmware. I am  just confused on which one to use. Can I use any stock firmware as long as it has been made for the GT-I9300 or should I find one which is specific to the region of my phone?

If so, how can I detect the region of the phone?

Phone is unlocked and International version.
You should use the latest firmware pertaining to your region and model only
Authentic source for all Samsung firmware is Sammobile. You will need to register(free) to download. If you want a faster download, there is a paid option. Make sure you select the right region
For detailed information on Samsung devices (including region as asked) Phone Info Samsung is highly recommended. Developer of this app has an active thread at XDA