Induction heater? I assume the coils are just in a plastic case for storage and not used like that?
I can see the coils are squashed up, they really need a slight gap.
I got my sleeving off amazon, its all about the same. Needs a bit of respect and will melt if it hits 200c, keep the heated part away from the sleeving.
Glass fibre with silicone coating usually. You may be able to get plain glass fibre, then it may be around 800-1200c but I imagine it would fray very badly and not be very durable. Perhaps a wrap of exhaust glass fibre wrap could go inside the sleeved coils for added protection.