Iris sensors at Lowes is currently on sale for $24.99, add two items to your shopping cart plus a small item like lubricant (we can always find a use for that!) Use store pick up. If you ship, then you might have to buy a higher price item to get free shipping for spending over $49 before discounts.
Or get some BlueStik putty to mount the sensors to your wall. Thanks for tip @johnconstantelo