As others have said, the shipper provides the placards.
That said, some drivers like to carry extras with them. If the shipper happens to be out of placards, haveing some of your own can save the load and avoid a canceled run. That has happened to us, but we also take an extra close look at everything else the shipper had done on that load. Being out of placards that a shipper is supposed to have is a red flag. If they messed up that part, they may have messed up other parts too.
We always ask for extra placards from shippers, in case one blows off or is otherwise damaged. That gives you the ability to build a placard collection over time. Just keep them in a secure pouch or container. If you were in an accident and your placard colection was strewn about, emergency responders would not likely go anywhere near your truck until they figured out what was in and not in your truck.