Auto Zone carries them. One of the "discount" parts store, carries probably 6 different configurations of the knife switch.
If you have electric steps that go up when you shut your door, you'll have to fool the switch. At least our batteries are under the inside steps. So if we open the knife switch, the steps will not go up. I have a mechanics pick up magnet, smaller than a ball point pen, that I put against the door mag switch, the steps go up, and then I open the knife switch. Close the door and all set.
Bill N8HDW & Nancy N8HDV
2005 Sea Breeze 8341 on W-22 Chassis
2008 Saturn Vue I-4