I bought two bottles of Quink a while ago (a black and a blue) and still have them. I even divided each into smaller bottles so I could keep ink in my old office and at home and all the jars are
still full.
So it lasts a while; and you can get the ink the same place you get the pens.
I usually get a big bag o' pens from the dollar store.
I used to like those, but I've noticed that they almost all now have the goofy click mechanism (where you have to push the clip in to retract the tip). It drives me nuts. I also end up with more pens than I can use and most get lost...
Love your enemies!