Electrical bench power equipment. Many power switches have a safety lock key that can be removed to disable the equipment. This is typically on a red power switch that seems to universally used on this type equipment. I have misplaced one. The question is:
Where can you purchase this simple device? Home depot does not have it available & recommended contacting the manufacturer. There must be a local source. The device is too common.

Check an electrical outlet, and some hardware stores. They make these switches for entire rooms, or just one inline with the machine. They will work with just about anything.

How about a sample tool model #?

Response to KISS
Delta Bench Belt Sander Model # 31-460 Type 2

Emergency Stop Button (http://www.electronics-manufacturers.com/products/emergency-stop-button-323.html)

Here it is: eReplacementParts.com : Key [1343758] - $3.61 (http://www.ereplacementparts.com/key-p-82886.html)

Used Parts & Accessories | Shop & Find Lawn & Garden, Appliance Parts at Sears PartsDirect | SearsPartsDirect.com (http://www.searspartsdirect.com) first.

And, your right. It's also used for other manufacturers.