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

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

I'll go straight to the best way to get more shift codes: The official 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 bot 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!