I was getting the same error in my IBM Websphere with c3p0 jar files. I have Oracle 10g database. I simply added the oraclejdbc.jar files in the Application server JVM in IBM Classpath using Websphere Console and the error was resolved.
The oraclejdbc.jar should be set with your C3P0 jar files in your Server Class path whatever it be tomcat, glassfish of IBM.