Anyone know what this insulator sheet is called?

I'm currently repairing my SoundStream Reference 1000sx amplifier and there is an amber colored, semi-transparent sheet that is sandwiched between the huge Darlington array, the power mosfets, and the amplifier's heatsink. I know it works as an electrical insulator and to allow heat transfer, but i'm having a total brain fart as to what the material is actually called. There was also white thermal grease on all the components to help aid in heat transfer.

I'm looking to either replace it with a new piece or if there is another, more modern material that transfers heat better, perhaps i'll use that instead. I already flattened the inner surface of the heatsink for better heat transfer and it currently has a 1200grit surface. So.... comments? Suggestions? Thanks in advance.
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Depending on the shape you might also be able to find a SilPad that'll fit. In the past I've bought SilPad in sheets from Digikey/Mouser. There are many similar thermal interface materials. Just beware that some of the silicone pads are both thermally and electrically conductive. You usually don't want the electrical conduction so check the data sheet before buying.

Like Kapton tape? but a sheet without adhesive? Here is a photo.

The piece is pretty big. it's approx 150mm x 300mm.
Kapton MT. The Kapton sold on ebay is typically Kapton HN and not as good. The MT is sold by McMaster but it's about $25/ft^2.

Unless so much of it is damaged that there is not enough left, I'd use what you have.

Yeah. i think i'm going to clean this up and the area where it's damaged where the blown Mosfets are, i can just use some of the remaining sheet where nothing is touching. I've also got a roll of good Kapton tape.

McMaster is an amazing place. Their HQ is about 15 minutes drive for me. The warehouse is so big it's like the scene at the end of Raiders of the Lost Ark where its shelves and boxes as far as the eye can see. lol.