I was wondering if anyone knows of a company that sells an external battery pack for the canon 420EX flash. Thanks

If the 420EX has a connection for an external battery, Sunpack and Quantum should fit the bill.
Mark.

The 420EX does not have a socket for accepting an external power cord. However, some companies, like <a href="http://www.qtm.com/">Quantum</a> make adapters that fit in the battery compartment.

This <a href="http://www.burren.cx/photo/flash_power.html">link</a> is a DIY solution.