This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Database drivers provide the implementations. Most of the database driver jar files will only contain the compiled class files. But if you look at hsql driver, it should also have the java source files.
Mind if I ask, what do you need it for? The purpose of JDBC is that you never need to see the implementation since it can vary among databases. Are you attempting to write your own JDBC driver? Is there a specific database server you want information about?