You could hunt on eBay for Rotring rollerballs. The Rotring Freeway, for instance, can be found for a song right now. And it's ridiculously good.
Rotring and many others take these Euro/Schneider/Schmidt style refills. Here's one retractable option: the Faber-Castell eMotion ballpoint will take this type of refill. I actually use OHTO needlepoint refills in mine and it works like a charm.
There are so many options, the problem is not the refill they take, rather it's choosing the body style you like.