If you use the Marketing Automation feature, the list of possible lead sources matches the titles of your marketing campaigns.
Administrators can enable this feature at Setup > Company > Enable Features > CRM.
If you do not use the Marketing Automation feature, in the UI, a user-defined lead source list is defined at Setup > Sales > Setup Tasks > CRM Lists > New > Lead Source.
Note:
The Marketing Automation feature must be turned off before you can create a user-defined lead source list.