I want to make my own plastic pieces but I don't know how to make the mold or how to even get/make the plastic.Ive seen the milk and vinegar thing and that plastic was really terrible and hard to work with.
What kind of plastic? There are numerous cold mold plastics which are fundamentally a resin and hardner. Epoxy is one of them. You usually mold them in latex, which you make out of an original. Another way is 3D printing. You have no mold as such, but print your item, either by ejection of heated thermoplastic, by printing hardner/binder on resin powder, or uv laser curing a resin. The popular hobbyist grade 3d printers use the ejection method, and can be user manufactured to a degree.
