It's always been a challenge to get the right models for printing. You may get lucky and find what you want in a freebie repository; you might find it for sale in a 3D print service's library, or you might have to resort to designing the object yourself with a 3D modelling tool - if you have the training and talent.
But now there's a completely new way to get just the right model: use i.Materialise's Sketch to 3D service. It's very straightforward in concept: you simply produce 2D perspective drawings of your desired item and submit them to Sketch to 3D. They say:
But because few can 3D model it has been impossible for most people to make exactly what they want to make, until now. The democratization of manufacuring has now reached you. Now you have no excuse, you can now 3D print whatever you want to.
How does it work? They're working with CAD modelling service GrabCAD to convert the sketches into actual 3D models. For the relatively low price of €80 (USD$80), they'll interpret your sketch into a true, printable 3D model. The rest of the process (including pricing) is i.Materialise's normal printing routine.
You can prepare a drawing in any drafting or drawing tool you can get your hands on, or even just scrawl it out on paper with a dull crayon. The catch is simply that the modelling dudes at GrabCAD must be able to understand what you want and how big it should be. But that's a heck of a lot easier than learning 3D modelling on your own.