AW and Hitting the help button in an alliance. You may get them in AW winner/losing crystals as well or the occasional milestone perhaps but all alliance related unfortunately as far as I know.Most places don't bite so take a dive!
Id suggest joining a low level alliance that doesn’t care if you log in or not for just ten days and just doing the Help option. They won’t expect anything from you, won’t bother you for BG or war, and you can get that gem.