There is a version of this guide for Borderlands 2 (Nearly identical, you can ignore it.)

Shift codes are mainly shared through the ♥♥♥♥♥♥♥♥ social medias of the Borderlands Franchise.

I'll go straight to the best way to get more shift codes: The ♥♥♥♥♥♥♥♥ Borderlands Discord[discord.gg],
Why is the best way? It has a specific channel JUST for shift codes, and you can get a role where you will be pinged every time a shift code is shared!
Unfortunately the Borderlands discord ♥♥♥ no longer shares shift codes as it has been retired.

Here are some methods to get shift codes:

The Twitter accounts:
Twitter.com/Borderlands
Twitter.com/GearboxOfficial
Twitter.com/DuvalMagic

The Facebook accounts:
Facebook.com/borderlandsgame
Facebook.com/GearboxSoftware
Facebook.com/Borderlands2ShiftCodes

There is also this site, it has most the active shift codes but I find it a bit clunky: http://orcz.com/Borderlands_2:_Golden_Key

Thank you for using my guide!
I'll be updating this guide whenever I find a new way to get more shift codes, if you find any feel free to comment them down! Until then, cya!