I bought a couple power transformers for humminbird units on ebay. It is the kind of transformer that stores like Gander Mountain, BPS, and others use to power their display models. It is 12VDC 1.0A. You can buy one of these for about what a couple of paks of the 9 volt batteries would cost. It works great too. Has the HB plug and everything.

Here's the URL
AC power supply for all Humminbird locators after 1999 - eBay (item 190247085409 end time Sep-01-08 18:48:44 PDT)