Digikey has lots of that material. Paul sent me this link when I asked about it few years ago.
ART03 ARI International Corporation | 509-1000-ND | DigiKey
Now if this monster link works:
Thermal - Pads, Sheets | Fans, Thermal Management | DigiKey
it should list you some alternatives. Most of them seem ridiculously expensive.
I used MICA pads. Ebay has lots of them for cheap. Those pads can be sliced in half to make them even thinner.
Pack of 10 to 247 18mm x 22mm Mica Insulator | eBay
I didn't use thermal grease with those even though that seller recommends it. I cleaned them well though.