Also, you have a very good chance of getting Iss from the daily's also.
The fastest way would be to use the mobile app and log in every time there is an event... But due to a horrible app you are forced to find other ways to AFK farm. There are a few options available to you like setting up custom triggers for ACT on zone events, or using a log program such as logview4net or kiwi log viewer to parse the NCL (non combat log) and notify you when a zone event has started. From there you have the option of routing your computer sound to your phone using software or even something like a FM transmitter (most android phones have a FM tuner built in) to notify you when you are AFK. Using this method I have gained over 200 ISS in a day with little overall effort. My friends have reported 250+ and I am sure double that could be earned if you had all day. Hope this helps.
Rifts drop ISS's pretty steadily these days. I'd put them on par with IA's in terms of drops per hour, though obviously you'll find more people doing IA's than you'll find willing to farm rifts for any length of time.