Jeff, As I have already explained to you Scan Colorado frequency directory with a yellow cover is a very good source of PL's. Additionally there are only about 3 public safety frequencies along the front range like Gilpin and Morgan County 153.845 and a few others where it really makes any difference. Also really easy to find the pl's with a tone finder but I truthfully don't use them for most of my VHF monitoring.
They were always available at the Newslands in Aurora and Westy but I think some of those stores have closed. Barnes and Noble had them up North but the directory is in revision currently will a new edition should be out soon.
Private Line they different users on the same channel.
But as I told Jeff at the beginning of this post, you don't need them for most public safety agencies in Colorado because they don't share the same frequency with others.