What transducer works with this unit? What is the additional cost?

There are several transducers that will work with this unit. Your selection will depend on boat type, how you want it mounted (thru-hull, transom mount, in-hull) and what type of information you want to see. Here is a list of compatible transducers for this unit, https://www.garmin.com/en-US/c/marine/boat-sonar-transducers/?productFilterTransducerCompatibility_GPSMAP=gpsmap_7x3xsv_9x3xsv_12x3xsv. If you'd like to have a more in depth conversation to choose the correct transducer, please give us a call! 866-735-5926.