There now exists an easy way to find all of a persons forum posts. Simply click on "View Activity" to the right of "Threads / Posts" in their profile. This page lists the threads the player has posted in in order of most recent post to oldest, one line per thread, on each line there are numbered links that will take you to each individual post.

Please note that you will only see HQ posts for those that are in your crew and you will only see Business District posts in the city you are currently in.