Reddit mentions of Schmidt 888 Safety Ceramic Rollerball Refills - Black - 12 Pack

I'm going to guess that this refill (what you're calling a cartridge) is a standard rollerball refill -- but it would be great if you could take the pen apart and post a picture of just the refill. If it is, it will likely fit in most nice rollerball pens (most capped, but a few retractable options out there).


An example of a standard rollerball refill (which is basically the same size as a Pilot G2, Pentel Energel RT, etc.): http://www.amazon.com/Schmidt-Safety-Ceramic-Rollerball-Refills/dp/B0037U3KH0/ref=sr_1_10?ie=UTF8&qid=1451872520&sr=8-10&keywords=rollerball+refill


Also, just so you know, what you're calling the "shell" is the pen body. :)

The best and smoothest writing rollerball refill, IMO, for the Retro 51 is the Schmidt Safety Ceramic Rollerball 888. It is available in several sizes and colors and is cheap.

That makes a lot of sense. I definitely don't want a pen that will ruin my clothes.

Edit: Do you happen to know if they will work with the Schmidt 888 ceramic ball refill?

The thing with rollerballs is that for the most part, the better ones you're going to find will be refill-based. If you're looking for budget pens, though, there are some decent options:

Bic Grip Roller: I normally don't care much for Bic pens. They are very much about the lowest-end of ballpoints for purely utilitarian purposes (and that's OK, just not for me) but I went to a conference earlier this year and got a Grip Roller from the Drexel University booth. It's comfortable to write with and has nice liquid ink. The plastic feels a little cheap and it's very light, but the rubber grip is pretty nice. And it's only $9/12 pack.

rOtring Tikky Rollerpoint: I love the Tikky rollerpoint. Read more here.

Stabilo Bionic Worker: This is a nice wet and inky rollerball with a super comfortable grip. Could be a little... ostentatious if you're using it for business meetings, but would be great for giving you nice bold lines for school or office work.

Pilot G2/Montblanc mod: This gets you a nice rollerball refill inside a cheap pen. I've been warned that this dries out quickly, but I've had it a few weeks and still writes pretty nice.

Refills:

Schneider Topball 850: I first got this in an Adler Stratas which I'd highly recommend because it's super comfortably to grip and write with, but you have to buy them in minimum lots of 50 since they're promotional. But they come with the 850 loaded in them, and it writes really nice. I'd imagine most rollerball pen bodies would fit this, and you can get them really cheap online.

Schmidt 888 Safety Ceramic Roller: I got this one loaded in a souvenir pen my father brought me back from London, and it writes really nice. I just checked and it's the same size as the Schneider, so it will also probably fit most refillable rollerball pens.