I thought I'd try the Ohto fountain pen in a drop last year. The drop was a good deal for 2 fountain pens, so one for a friend and one for myself. We both really like the FP's. So, when the drop for 2 Rollerballs came up, I ordered them. Very pleased with the rollerballs as well. Ohto makes it easy to tell which is the rollerball v. the fountain pen - The finial/top of the FP is rounded and the top of the rollerball is flat. So if you're going to store them in a pen pouch or bag, you'll be able to pull out the right one easily.
Sallen - Jet Pens does sell the refills in black, blue, blue black and red. They offer 0.5 and 0.7 sizes @ 3.30.
( Jet Pens offers free shipping for orders of 25.00 or more. Which isn't hard to reach if you love good writing supplies)