What is the etiquette for asking a professor I don't know to suggest universities for me?
I would advise against it.

I don't think most people have in their heads a list of "good universities in my field".  When advising students who they know on where to apply for grad school, they try to recommend schools based on everything they know about the student, their interests, talents, constraints, etc.  For a student who they don't know and have never met, this would be hard to do effectively, and I don't think most people would really be interested in trying.

If you send such an email, I suspect you will either get no response, or a brief "Sorry, I can't help".

Instead, I would suggest that you talk to a faculty member in your field at your own university, someone who knows you well and whose opinion you respect.  They would be a much better source of advice for you.