Is it the actual transducer piece you're missing, with a cable running to it, that sits down in the water? They're not standardized at all, your best bet is to either find parts from the same manufacturer, or get another fish finder as you're saying.Actually, I'm glad someone else necro-bumped this.
@gee, I got given a '60s era fish finder, the instruction manual has the circuit diagram of the whole setup. One of the problems is that it's missing one of the transducers that tells you what the depth is. I have one, but it is supposed to have a spare. I'm wondering if you might be able to help me figure out how to get another one without cranking out $60 for another fishfinder of similar vintage.
Figured you might be interested in the circuitry anyways.
Also, a modern fish finder with DSP will kick the shit out of that old stuff.