My everyday pocket pen for the last year or more has been one of the Rollester rollerballs from PSI. It is not without it's issues though, primarily the fit and sometimes straightness of the snap on cap. I got mine in the starter set from PSI for about $6.75 per pen otherwise the cost just under $8.00.
For the two-part (more traditional) rollerball style, I prefer Smitty's Caballero kits, now available from Ed at Exotic Blanks. Most plating options are available for about $13.00 or so. Ed also carries the Diamond Knurl Rollerballs for about the same price. I also use less expensive Jr. Gentleman kits from Legacy Woodturning (The Woodturning Store) for about $6.00; however, the quality or at least perceived quality of the Caballero is worth the difference in my opinion.
One of the things I do with almost all of the rollerball pens I make and use or give away is to replace the refill with a gel ink rather than the standard liquid rollerball ink refills. My favorite is the Pilot G-2.
Regards,
Dave