Plastic injection is very expensive because the molds are very expensive to make. There are extream pressures involved with plastic injection molding so the molds need to be CNC machined from steel. CNC machining has always been very expensive to do. If you are in a money no object situation you can look in the
Thomas Register to find a manufacturer that does plastic injection production.
There is low volume plastic injection molding available too you can search for that have lower startup costs.
What do your products look like? Are they small? If small you can get them spin cast in a number of materials at reasonable costs
When talking about molds you can make yourself, checkout Smooth-On, in particular, their
tutorials section. There are all kinds of ways to make your own molds using Smooth-On products.
If your question is about the 3D modeling aspect
@andrewsmodels explanation from years ago using Zbrush is one way, but traditionally solid modeling software is used, software like SpaceClaim, Solidworks, Solidedge. This kind of software lets one easily design in draft angles in the walls of your mold that allow the ejection of a rigid part.