I had downloaded some shader packs before, but now I can't find my shader packs folder. I am on a Mac.
The shader packs folder on a Mac is
~/library/application support/minecraft/shaderpacks. (
~ stands for your user folder and will be automatically replaced with your userfolder when using ⇧+⌘+G)
However, you can also get to the shader packs folder by going to
Options... > Video Settings... --> Shaders... --> Open shader packs folder.