Gatorade, deja vu! I asked exactly the same question in exactly the same circumstances a few months ago

See here ...
http://www.fountainpennetwork.com/forum/in...5241&hl=rotringBut to cut to the chase, it seems that Rotring do make (or at least sell) their own ink, but no longer in bottled form. You can still get the cartridges though. I got mine from a UK supplier, so the link probably isn't much use to you. But if you can't find any in the US, let me know and I'll look out the link.
HTH
RichardS