Where to get them... Clicking on an unexplored piece of land around your estate will allow you to post for clues
or you can look for wall posts from your neighbor's that give out clues...
(this filter shows ALL the HC posts https://www.facebook.com/?sk=app_100333333405439 from your newsfeed)
Zynga has put limits on how many clues you can collect off your wall each day
(currently it is 5 of the "Send Clue" and 5 "Get Clue" Every 24 hour)
((Trying to collect more than 5 will give you the message "This reward is too popular" even though the post might not be out))
Try and post for them every chance you get. You can hold up to 35 Clues in your inventory.