There is currently no public API for getting the javax.sql.DataSource instance for Apache SIS SpatialMetadata database. There is Initializer.getDataSource() method in the org.apache.sis.internal.metadata.sql but this is non-public API, subject to change in any future SIS version and will not be accessible in Java 9 (unless the restriction is bypassed by Java options on the command-line).
A standard way to provide DataSource in Java is with Java Naming and Directory Interface (JNDI). However there is no JNDI environment by default in standard Java installation; JNDI are rather provided by containers like Tomcat. We could provide our own micro-JNDI when no environment is installed, but it may be disruptive as installing our own JNDI environment would have a JVM-wide impact.
A last-resort approach would be a public static DataSource getXXX() method somewhere, but it would not follow standard mechanisms (JNDI, dependency injection, etc.).