Deadstock fabric is leftover fabric from clothing manufacturers. Deadstock fabric can actually be any kind of fabric. As it is not being used for its original purpose, it can be considered sustainable. However it has always been the case that deadstock fabric very rarely ends up in landfill, surplus fabric is nearly always used by another manufacturer or is made available for sale for dressmaking.
The great thing for you is that generally these fabrics are great quality; are only available in small quantities and very good value for money!