What is......

[Ringel05]

Tailor's felt and can regular felt be used as a replacement?

 

[Luddly Neddite]

Its used for interfacing and must be able to take "tailor's chalk". There's also "tailor's linen" and another, the name of which escapes me.

Depending on the project, I would think there are a lot of possible substitutes. You can also use white prismacolor or soap slivers instead of the chalk.