My tile bathroom floor has been built up so it is not lower than the flange. Water leaks out slightly when the toilet flushes. Is there an extension piece for the already existing flange or do I need to get a different, higher flange?

You can replace the flange but they do sell a flange spacer for about $2 at Home Depot. You finished flange top surface should be flush with the floor top surface or sitting on top of the floor surface. Some folks try a double wax ring and claim success but I'd buy that spacer.

Is there an extension piece for the already existing flange
There sure is Ryan,
Purchase a set of flange extenders,(see image) and a waxseal with a funnel, (see image). Pull the toilet and clean up the wax. Using the extenders build up the flange to floor level and reset the toilet using the new wax seal. Good luck, Tom