Old work boxes for lights

I want to add two electrical boxes to the wall in my bathroom for 2 sconces. The sconces aren't very heavy and where I need to hang them is not next to a stud.



I am having a hard time finding old work octagon type boxes at local stores. Is there something else I should be using? I've seen plastic ones online, but I don't think those are up to code where I live (Chicago).



I've seen those boxes with the bars, but those seem more for ceilings.

Old work boxes for lights