Is there a workaround to get the list of organizations on GitHub?
For example: https://github.com/showcases/open-source-organizations
How can we do that via the GitHub API or GitHub search?
You can get a list of all accounts:
https://developer.github.com/v3/users/#get-all-users
The type
parameter will tell you if it's a user or organization.
An alternative is to use the search API:
https://developer.github.com/v3/search/#search-users
You can specify type:org
to get organizations only: