Stopped by my local Office Depot (here in Southern California) for other reasons and checked their refills for Barons. They have several that look like they are the right size and shape to work but here are the three that I have been using. They are rollerball refills called 'Pilot Gel Ink' and they have a fine tip. The part numbers are below.
526-550, $2.09 for 2, Black
526-587, $2.09 for 2, Blue
526-596, $2.09 for 2, Red
I just had a look at their website (www.officedepot.com) and the black and the blue refills can be ordered off their website (just do a search for a part number). The red ones didn't show up but they have them in the store here.
I'm sure there are many brands, styles and sources out there but this should give you a good start.
Todd