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.
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