Here it is:
http://www.radioshack.com/product/index.jsp?productId=2103641&cp
It's basically the same thing that was pointed out above (the build your own).
I think I bought mine on sale at some point.
You will need an adapter to go from the "port" on the bottom of the antenna to the coax style.
While I was looking it up, I realized in the new house, I am not using that. I am only using the 1/4wave whip that came with the W800. My LAST house it was NECESSARY (I had metalized insulation). This house...I can get DS10's from 1.5 houses down the street...using the 1/4 wave. I did use it at the last house, and I went from barely able to get a KR15A(?) from the edge of my property to 2 or 3 houses away.
--Dan
(?)it is the security keychain remote from Radio Shack...same as the X10 version, but nicer form factor and 2 light controls besides the Alarm / Disarm)