I don't think these take standard international cartridges though. I tried it and it doesn't seem to work. It said it uses Pilot Ink cartridge UIC50BLU-K on the package.
Interesting--you found a Plumix at Target? What town/city/state/country?
I just discovered this thread, and thought I'd toss in $0.02:
I bought a Pilot Plumix last Friday at the Target store on High St just north of Morse Rd in Columbus, Ohio. I didn't go there looking for a cheap fountain pen, but I found one. There were three on the peg hook - if I'd known what I was buying, I'd have bought all they had.
I sure hope a PR Spearmint cart goes into it - I just ordered a box of them from isellpens.com, and they haven't been delivered yet.