A Google search for ["police patches" forum] brings up these, among others:

http://forums.realpolice.net/forumdisplay.php?f=13

http://patchtraders.net/phpBB2/portal.php

http://www.policeworld.net/vb/archive/index.php?t-3336.html

Try searching Google Groups for newsgroups too... here's one I found:

http://groups.google.com/group/alt.collecting.police-patches

Have fun!