On the moral fitness form in CA, they ask for a personal reference that is a practicing attorney. The only attorneys that know me well are the ones I worked for and I assume they will be contacted as part of legal employers.

Can I use a law professor as the practicing attorney reference for a personal reference? Or should I write them a letter explaining the only attorneys who know me are already on my forms.

Use a professor at your school.  At my school we had a dean that was a student advisor for our class who told us to use her as a reference.  Your school might have a designated reference.