You can use just about any NMEA GPS unit out there that has a true RS232 serial interface. Check eBay or a hamfest for older Garmin or other brands, especially if they include the cable. I saw a Garmin GPS-III at a hamfest a month or 2 ago with the serial/power cable for $30.
I use an old Garmin GPS-III that I have had for most of a decade, While it is pretty much useless for navigation these days on it's own due to the outdated maps, it still works wonderfully for providing GPS data to a couple scanners, my dual band APRS rig and occasionally a laptop with mapping software.
I leave it powered 24/7 in my van. It can run for months without running down the car battery, and this way I don't need to wait the 10 minutes or so it sometimes takes to acquire satellites or remember to turn it on.