One of the best places is the official Amibroker Library
http://www.amibroker.com/library.html
several other links
http://www.wisestocktrader.com/indicators/
http://atma-india.net/component/option,com_kunena/Itemid,10/catid,29/func,view/id,1451/
http://www.marketcalls.in/library
http://www.inditraders.com/amibroker/129-amibroker-simple-useful-codes.html