Hi Oil-Rig Mike,
You can get them from Sling, or you can get them from somewhere else (like eBay or a good home theater shop).
If you get one from somewhere else, you may need an adapter so the plug fits in the hole. Look for a 3.5 mm mono Female > 2.5 mm mono Male adapter. Radio Shack sells them for a couple bucks.
Here's a link to the one Sling sells:
Hope this helps,