I don't know what it actually is but it looks like a Green Scouring Pad. Of course the one you use on Pans are only about 1/4" Thick.

I can't tell from the pic if the Color is from what was filtered or the natural color of the Material.

If it is actually a thick Scouring Pad a Restaurant Supply store, McMaster Carr or the name escapes Me but there is another Company like McMaster Carr.
Maybe Janitorial stores?

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84 300D, 82 Volvo 244Gl Diesel