I think once found that kind of suit, it was fabric impregnated with rubber (very think and heavy material)
colors may vary, mine was orange same suits are often used by sailors.
if it was military clothing i think color may be brown/green like usual
so you probably can just buy that sailor coat and color it in proper dark color somehow.
It was probably just leather. Early equivelanfs of biohazard suits were made of it (trench coats in WWI were worn specifically to protect from blister agents), and most chemicals weren't understood enough for sophisticated protective suits to be worn (for example, plans for the invasion of mainland Japan had the US dropping a bunch of nukes and then marching in troops wearing gas masks and trench coats for "protection").
Leather is not very good choice as it can easily suck those agents inside and you will not be able to wash them out . It is not impenetrable as rubber and very expensive.
If leather was ever used it must be very very old times when rubber was not available.
look for "death by vaginal cockroach" in efukt.com