This is the ink I bought online, when I bought the pens. Accompanying it is a piece of envelope that was one of my improvised funnels, and the paper plate that turned out to be a woefully inadequate protective surface.
Here's a picture of the quality I got out of the pens. Note that in several places the ink bleeds badly.
Here is the solution to all the problems above! Higgins non-waterproof India ink, and a baby-medicine syringe I got at the drug store. (The pens hold just about exactly 8 mL of ink. Or, in fact, any substance.)
And here's the pens themselves, on top of the heavily doodled test page, which, you may notice, features no bleeding whatsoever.
(If you're interested in deciphering it, it should be pretty much nothing but the lyrics to "Mr. Me" by They Might Be Giants, upside-down.)