Confused about thermal insulators for TO-220

I bought some insulator strips for MOSFETs. They prevent the heatsink from being electrically active by conducting heat but not electricity. Yet, I still use a metal screw to attach the heatsink to the metal tab on the MOSFET. So, isn't that going to conduct current? Are you supposed to buy special screws too? The strip has a hole in it, which suggests it is for use with screws.

An insulating nylon shoulder washer fits into the hole in the TO-220 package to insulate the screw. Some people use nylon screws instead but they break.

I see, so I need a washer with a sleeve that extends into the tab's whole. My heatsink uses 3MM screws, so I assume the hole must be 4 or 5 mm?

Does anyone have around 10 of these they could mail me in an envelope for payment using PayPal? It looks like they cost $0.08 at Newark, if you happen to want 1000 of them. Aavid part number 7721-7PPSG, or similar.

I have a bunch of nylon shoulder washers for TO-220, you can have 10 for the cost of postage. Where are you?