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

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.

The modern alternative to mica and grease is the Sil-Pad products from Bergquist and others. These are not very expensive and they're a lot more convenient to work with.

For very good thermal performance and isolation, go with Bergquist Hi-Flow.

It's phase change material that goes liquid-ish above 55C, filling whatever voids are necessary, plus it has a Kapton insulator in the middle to give you good dielectric strength. It works best with clip-type connectors that hold constant pressure, otherwise you need to pre-bake the heatsink and torque down screws a second time once the material flows.