Self study. I'm making a course.
The test costs $100 and does not require that you have taken a course. I did a 3 day course though, but also self study. The course was not necessary to pass the test.
Start with:
1.) Read the Scrum Guide.
2.) Then go to Scrum.org
3.) I like this kid's "Firehose" overload course: Scrum Firehose Overload Version
4.) Once you think you are ready, do some practice tests: Practice Tests
5.) When you're ready, do the test for certification ($100): Scrum Tests
Good luck!
Thanks!