Depends what temp/pressure you are talking about, I used to use Golden Walkerite or Klingerit 1000 for steam joints over 1000*F and 2000psi. available in several thicknesses,There is probably a gasket material merchant on an industrial estate near you,they might let you have some off-cuts depending upon the sizes you require,
How crucial is it exactly I get the correct thickness for these gaskets,as its difficult to get an accurate measurement after being crushed for god knows how many years.
Any decent jointing material will not have compressed more than a couple of thou" Mic' up the old joint and it will most likely be near enough the thickness you need to replace it with.