Try this for MPD
https://oap.mpdconline.com/public/ContentPages/Reserve.aspx (broken link)
This for Montgomery County PD
MC Department of Police: 1st District
Basically just google the name of the police department and 'volunteer' and that should show you pretty quickly if they take volunteers. I hear the best departments to work for are Montgomery County and Fairfax County, so I imagine they're also good places to volunteer.