Accessory plug will accept NMEA 0183, a standard for many GPS units. Try a Garmin with NMEA 0183 output.
Data exchange is RS232, probably 3 wire and may need a null modem.
That's what I was afraid of. I did'nt want a GPS that cost more than the receiver. Lesson: You get what you pay for. It would have been nice if the receiver had bluetooth connectivity so I could have my iPhone supply GPS to it.
You can find Garmin GPS awful cheap, even with 5" displays. However, marine maps from garmin can start getting expensive.
VHF, at least, all you need is coastal and maybe inland water ways.
[SOLVED] So I purchased a BR-355N GPS Mouse from Ama... for $60 (CDN), chopped off the connector, wired it to my RX and added a 5v power supply I had hanging around to power the mouse and... Bob's your Uncle. The UM385 now gives me a "GPS data OK" on the screen.