First, I confess that I'm not a "touring" cyclist. That being said, I figured that the touring crown might be the most knowledgeable when it comes to cassettes other than the standard 12-23 road cassettes. I'm looking for a source for a quality 13-28 to replace my 12-25 Dura Ace cassette. Any help is really appreciated.

As long as we're not talking 10-speed you can use a wide range Mtb cassette but your road derailleur will not be able to handle the larger cogs. You would most likely have to swap to a Mtb derailleur. Switching to a triple ring setup up front may not be a bad idea as well.

check out Sheldon Brown's page on cassettes/freewheels. They (Harris Cycles..?) put together a few custom Cassettes in ranges that arent off the shelf available.. I think they actually do have a 13-28 setup!!! nice stuff..
~Steve