All suiting fabrics are a great choice for your waistcoats. Any fabric shop that’s worth your business should have a suiting section. You can just walk in, ask where you can find the fabric for suits, and pick out something you like.
These fabrics are typically wool, but you can also be a bit more creative and go for something else.
You do need a bit of weight though. Denim is fine, some light cotton is not.