In addition to Mikes answer in The Hobbit, the coat Thorin chooses for Bilbo is described as
"...wrought for some young elf-prince long ago. It was of silver-steel which the elves call mithril"
For it to fit Bilbo it had to be a very young elf prince. The idea that there was only ever one mithril chain mail shirt forged and it was given to young elf who would soon out grow it seems very unlikely.