Here are multiple thoughts.

The Worldwide Pinhole Photography Day website has a resources page with lots of excellent information.
http://www.pinholeday.org/support/

I use both purchased and home-made pinholes - and purchased and home-made cameras as well - depending on the need or use for the camera. For solargraphs, multi-month outdoor exposures, I use home-made cameras with home-made pinholes. I have a few series of identical home-made single-shot cameras, and I want all the cameras to have the same - same exposure times and quality of image. This can best be achieved with identical purchased pinholes. There is generally a Zero 2000 and a Pinhole Blender in my car at all times.

The best purchased pinholes are electron microscope slides. EMS slides were mentioned earlier in this thread. They have to be purchased in bulk; but it works out to be less than 30 cents (.3 USD) per pinhole.

The best material for hand-made pinholes is probably disposable aluminum steam tray lids (from Party City and similar stores). They are thin, thin sheets and for a dollar or so per sheet you can make dozens of pinholes.

A shameless plug: the next Worldwide Pinhole Photography Day is April 28, 2013. It is coming up fast, and home-made or purchased... join in!