What parts are needed to add a shutoff valve to 1/4" refrigerator water supply?
Saddle valves normally have a T handle that acts as a shutoff:



These are notoriously troublesome. The original one in my home crusted shut, as did the replacement by the previous owner.

In any event, you'll give yourself a more reliable service and vastly improved pressure with a proper valve at the supply pipe, such as those found near toilets:



Of course, you can always add an inline valve if you wish to keep things simple: