Get your Pie now

Sammobile.com is a reputable source that generally has updates pretty quick. Just have to be sure the file you're downloading is for your sub-model, go by the "SM-G960x" code in Settings...
 
G965usqu3csa7 is the build for pie, you have to be on BRK2 before installing it tho, but go to https://firmware.science/ put in your model number and then the build you are on now and it will show you your updates, the G965usqu3csa7 is pie...
 
That's not the official release from what I've read. I believe that's a leaked build for developers. I'm not sure you'll get OTA updates through it now. I guess it depends on your carrier.
 
stable Android Pie-based One UI update comes with build number G960FXXU2CSA2 and G965FXXU2CSA2 for the Galaxy S9 and S9+ Exynos variants, respectively. For the Snapdragon Galaxy S9 and S9+, it is G960USQU3CSA7 and G965USQU3CSA7 respectively. It has already started rolling out to the users in Europe, including Italy, Germany, and Switzerland. You can also force-check for the update by going to ‘Settings’ > ‘System Updates