I ordered some refill cartridges for my Uni-Ball Signo pen, and they arrived in the mail today. I was complaining that it looked like it was cheaper to buy a whole pen than the cartridge by itself, but searching deeper through Amazon I found a good deal. The trick was to use the name “Mitsubishi” in the search, which is the parent company of Uni-Ball. I had to go all the way to Japan for the order, but they didn’t charge for shipping. The estimated delivery time was stated to be a month, but wound up being only 12 days. The pens I have are .38mm, and these cartridges are .28, but they fit fine in the pen, and there’s no noticeable difference in the line.
(Note: It looks like they raised the price a little. My order was only $10.36 – still cheaper than 10 whole pens which cost $16 + shipping.)