Well, Police and Teachers are the stereotypical referee jobs in my experience.
Although I know most teachers will generally stay late to some extent, you can on occasion leave bang on 4, so I guess that works well for the occasional midweek game. And police will be shift work I assume, but organised much further in advance than a lot of shift jobs - again, giving you the chance to arrange matches when required well in advance.
I work 9-5 mon-fri, which obviously gives me complete freedom at weekends and is not really a problem for taking local midweek matches. I think as long as you're prepared to be flexible with what leagues you're refereeing on, all you need to worry about is avoiding a job that will have you working every Saturday.