I've made my own molds for vacuum formed plastic parts, and you can make an homemade machine to do simple parts that way. This is a pretty inexpensive route to go but it's also limited in what you can make. If you're creative you can build a decent prototype with vacuum form plastic and machined plastic parts. This can be a good route to go during the proof of design phase of a product.
Nowadays, for a demo product you can also look into 3-D printers for prototypes and small runs of your designs and this is probably what you should do before going into production.
For production volumes you need to find a "injection mold company". Google the term and you'll get a list.
I'd talk to matt_the_bass first if I were you though. He's in the biz and took the time to respond.