Nest season I will be directing a courtroom travesty and need costumes for the bailiff and prison guard.  I have been searching on-line for costumes to rent or buy but cannot find anything better than a "Sexy Officer Ginger/Steve" that is only one step shy of a stripper outfit. (granted it is a comedy, but I'd still like realistic costumes.  Especially since we're performing the show in the County Circuit Court Room. ) 

Check out a local Army/Navy Surplus store.  They might have useable items for less.

How about trying a local security company?  Ask if you can trade out insignia/logos/patches on their uniform shirt, and then return the uniform to its original state before returning to the security company.

How about simply calling your local police department and asking if they have old uniforms.  Most departments would like the good PR of having a thank you in the program!

I agree with teridtiger... even in our very small town the police department is unwilling to lend any of this type of equipment, I suppose for the same reason.  Still many shows it seems require a policeman to be costumed so you really need to hit upon something that you feel works for you.  We have done what Nanette suggests--- found navy pants and shirts that closely approximate the look and add patches.  Actually, we were able to obtain through a website an actual NYC police dept patch to sew on a shirt for Murray the Cop in the Odd Couple...which surprised us.