Solution 1: using Emacs
If you have an Emacs window open almost all the time, you can use Emacs to do it, on Mac or other OSs. Specifically, Ctrl-x, Enter, then start typing the character's name, namely, "
Solution 2: using Chinese input method
These characters are CJK book quotation marks. Many Chinese input methods map
opt-\ → «
shift-opt-\ → »
shift-opt-3 → ‹
shift-opt-4 → ›
Not terribly intuitive, but the keyboard viewer is your friend. You can also make it easy to switch the keyboard language to one that might be more familiar to you.