Your best bet is ebay. If I remember the 440S & 540S (and 545S) used the same plugs but they don't have a network port for a radar. You could buy a later model but you'd need to buy an adapter to change the 6-pin transducer plug used by the 440S. If you want to add radar, you would need to find a used 740S. It would use your existing transducer. You should be able to find one in the $250-$350 range.