(introish paragraph on exigence, audience, and professional writing)
I don't expect you to file and pay for Freedom of Information Requests, and it's understandably rare for people to post private or protected correspondence on the internet for anyone to access. While I would not simply say "give up looking," if you're not networked with a professional or have professional experience as an employee, volunteer, or intern, private documents will be very difficult to get hold of.
Thankfully, there are dozens of other genres that you can access when researching professional writing online.
Part of being a professional is participating in professional communities.
All kinds of organizations use social media and social networking sites for a variety of professional purposes. This is exactly the kind of distributed writing work that you might be expected to contribute to as an employee of the organization.
See an overview of Corpus Christi PD's social media and police-media relations.
Blogs are used in a variety of ways: as police blotters, as easy-to-update methods for publishing information to the community, as narrative storytelling forums (what role might that have in the profession, as a form professional writing?) or as informational and educational resources.
Local PDs publish Uniform Crime Reports in a variety of formats: