Lead Source can be set as required on Quote Templates. If you're not familiar with Quote Templates, watch the short video below.

To access quote templates:

1. Hover over Quote at page top, then Quote Templates.
2. Hover over Auto or Home, then Edit Quote Template.
3. Select the State (NOTE: templates are created per state, so update accordingly).
4. At the template, click Edit (update each template accordingly).
5. At the Lead Source field, set to Show as Blank & Required.
6. At page bottom, Save & Exit.