You can even do small mass production using 3D printers. For a few hundred/thousand pieces, it's likely cheaper to buy 3D printed stuff than to invest in injection molds.
There are moulding processes that are economical from a few tens of units. Mostly these involve making soft moulds from 3D printed forms and the moulds are good for perhaps 50 units each. They are slower though so if you want a hundred units quickly it might be better to print, and just contract out to multiple printing places if you need extra capacity to speed it up.