RFTools Dimlets
Let's start by saying you don't need any dimlets to create a dimension. You can just specify no dimlets and generate a totally random world. Maybe you get lucky?
However, to have any kind of control you will need dimlets. Scattered throughout the world, in dungeons and other chests, you have a chance of finding 'unknown dimlets'. These small items were made a long time ago with long forgotten technology. There is also a small chance that you can get an unknown dimlet from killing an enderman. However, the best place to find dimlets is in RFTools dimensions. You could go ahead, create a random dimension and go find an RFTools dungeon:
What do you do with these unknown dimlets? You put them in the 'Dimlet Researcher' machine.