I just went out and bought the Garmin GPS 18. I got it at Provantage. Go
here. That is the OEM version that doesn't come with any software. Also make sure you get the one that uses the serial interface, not the USB interface. If you want one that you can also hook up to a laptop get the normal version with software, again make sure you get the serial version.
They charge about $68 for the OEM version. I don't think you can beat that.
Personally I use the gps 10 bluetooth with my PDA and I bought the full version gps 18 for the 996 and my laptop.