The 3500 uses a Lowrance 192 kHz transducer with gray connectors. Any Lowrance head unit with gray connectors and a similar frequency would be plug and play. Many other manufacturers use transducers in the 200 kHz range, so they would work with your transducer, but would require rewiring the connectors to match.
The Lowrance 3800 is the newer unit and also has temperature. You won't get temp with your existing 'ducer, and the unit is about $175. The Humminbird HDR 650 is about $90, it is also a 2 1/8" gauge (so it fits the same hole), uses a similar 200 kHz transducer, and you should just be able to cut and reconnect the wires to your transducer to make it work.
Then you have HawkEye - just $69 gets it, and it will probably work with your 'ducer. But if it does not, you can use the glue-down shoot-thru 'ducer that Hawkeye offers. Great product for the price and with a 2-year warranty. Check it out at:
http://www.hawkeyeelectronics.com/depth-finder/ Good luck!