Steve,
You can buy one direct from us over the phone (1-800-747-9329) or over the internet (http://www.humminbird.com/retail/default.asp?catalog_name=Zercom). You can also find a dealer off the web site at: http://www.humminbird.com/hb_children.asp?id=54
Your least expensive option would be to send the transducer in for service. A new transducer will cost you about $50. Having one fixed is about $30 plus shipping (another $5). Of course if you just 'gotta have it now, we can ship it next day air but it will cost you. Call the Zercom 1-800 number and talk to the experts for prices and details. They are not in just yet or I would have checked. Oh, be sure and tell them if yours has the built in temp as some do.
Sometimes those stumps just get hungry!
Operator of one very used 1997 Pro Team 18DC