Currently, there isn't any prep content for subject tests (only for the normal SAT examination). This article addresses commonly asked questions regarding SAT test prep and includes some information about the SAT subject test. A link to YouTube playlists with content from Khan Academy useful for studying for the subject tests is included in the article (fourth question).

There is a lot of Biology course content on Khan Academy which includes videos, articles, and practice exercises.

Let me know if you have any further questions!