adding ruby gem sources

If you are getting an error like -

ERROR: could not find gem GEM_NAME locally or in a repository

then probably you might have missed the gem source if the name of gem is correct.

You can add a gem source link using command-
gem sources -a source_link

To view existing gem sources-
gem sources

Few gems sources-
http://gems.rubyforge.org/
http://gems.github.com