I need to create a flame barrier which must be thin (5mm approx) and
fairly strong. Asbestos sheet would have been ideal if only it was
still available.

What I need it for is to block a chimney off with it, diverting the
heat and fumes off to another flue. It'll be located fairly close
above - but not actually touching - the flames coming from a 'living
flame' gas fire immediately underneath.

Anyone know of a good substitute for asbestos sheet and where to
obtain it?

Thank you,
MJ