I could never find a replacement knob for my headlight switch (or it was only sold in conjunction with the entire switch unit), so I had one 3D printed. This might be a good option for anyone else having a hard time finding the right replacement part.
The original knob was missing a cap or something that covered the front. The car's previous owner had glued a piece of plastic over it, but it recently fell off. I measured the intact part of the knob and modeled a new cover in Blender, a free 3D application. The model was then uploaded to Shapeways, a 3D printing company based in NYC.
A week later, the plastic part arrived in the mail. There are a variety of different materials that you can choose to have the model made from. I chose "Black Strong & Flexible," and it turned out to be a pretty close match to my car's interior.
After sanding the back of the knob slightly to help glue adhere, I attached it to the switch body. It's been a few months since it was glued on and still seems to be holding up well. Having the knob printed only cost about $10 (with shipping) and turned out to be a pretty decent replacement. If there's a small part in your car that needs to be replaced but can't be found cheaply, 3D printing it seems like a good solution.