Custom shortcut for switching input sources in macOS High Sierra
Go to System Preferences > Keyboard > Shortcuts > Input Sources:



Then double-click the shortcut listed to the right and enter your preference.


References:
macOS Sierra: Use global keyboard shortcuts (Published Date: Mar 28, 2017)
Mac keyboard shortcuts (Published Date: Jun 11, 2017)
OS X El Capitan: Use input sources to type in other languages (Published Date: Sep 15, 2017)
macOS Sierra: If a keyboard shortcut doesn’t work (Published Date: Mar 28, 2017)