I buy my pens at an art and craft store. I buy them there because they have a wide selection of colors and are readily available.
As far as writing with them, on blanks or paper sometimes they can be a bit stubborn.

Evertime before I use one ,I make sure I have a good flow of ink coming out of them.(much like when a regular ball point is running low on ink and you scribble with it until you have a good flow). The things I like about these pens is they do not scratch the surface of the blank, mistakes are easily wiped off without marring the surface and the availablity of many colors.

Once your satisfied with what ever you've writen, the use of the fixatif (I mentioned in a previuos post) is sprayed on and holds the ink fast. Within a few minutes, you can handle the rod without smearing your work.