What material should I use to insulate my MOSFET from a heatsink?

I have a heat-sink and I want to put two FET's on it. I know I have to insulate it so the metal showing on it doesn't connect and create a path. Is there any good everyday material that would work? (ex. plastic bag) My packaging is TO-247 (irfp250/450) and I found 1 insulator laying around, but not 2 for both of them. So do I need to put the insulator on both MOSFETs or is one good enough?

Answer

Depending on the MOSFET package, mica washers are an option.

A common through-hole package for MOSFETs is the TO-220, where the metal tab looks like this from the back:

_{(source: Wikipedia)}

_{Edit: OP clarifies that their MOSFETs are TO-247 packages, for which the TO-220 insulators can be used - or TO-247 ones possibly, though those don't seem to be sold as a distinct product much.}

For these, mica insulator kits are available at extremely low prices, containing the washer itself, a nut, bolt, spring washer, and an insulating sleeve to isolate the bolt from the metal tab:

_(source)

Insulation is preferably done for each of the MOSFETs you are attaching to a heat-sink, to avoid undesired short circuits.

You should also use a bit of heat-sink compound between the mica and the metal tab, as well as between the mica and the heat sink itself.