Most transducers are made by Airmar then repackaged with brand names on them, and you probably could get one directly from Airmar for it, just call them and they will tell you what will work with your unit.....
That said the Humminbird XFM 9 20 should work for your 597ci...
http://store.humminbird.com/products...725c8567368874
93 bucks at the HB store and they say it works.... don't take my word for it check around before you buy.
Actually the cheapest places to buy transducers are usually computer discount supply chains online. I found one for 58 bucks but they had the wrong picture, and might not know what they are selling.
Better is 62.99 bucks here at nerds:
http://www.thenerds.net/HUMMINBIRD.H...1.html?affid=8
or..
63.99 at Poorfish outdoors
http://www.poorfish.com/p-9784-hummi...-xfm-9-20.aspx if you want to go with a company that sells specifically fishing stuff.
Or you can do it commando style and make your own thru hull:
Works for me..literally...
I actually miss the days when we had to do these things ourselves
Thru hulls are the only way to go...Any way you look at it your going to get much, much, much better performance with your transducer in the water like it's designed to work rather then beaming through the hull with grease or glue. It kind of blows my mind that people are now putting high performance fishfinders in their yaks and still killing their performance by gluing in thier transducers. To each thier own though.
Good luck!!!
Jim