The "NASA Space Pen by Fisher" is a cool pen that retails in stores like Staples and Office Depot for about $22 in its basic, chrome-plated version, up to near $50 for the special, "signature" version. It is the famous pen the astronauts supposedly used in the space program. The pen uses a sealed, gas-pressurized ink cartridge, which delivers the ink at a perfect flow without clogging--important for a pocket pen. The problem is the NASA Space Pen body is very thick. It feels like a carrying around a big, chrome-plated bullet in your pocket.
It's way too thick to slip in your wallet, too. Overall not a good traveling pen, even at that price point. But, interestingly, the same outfits that sell the pens also sell the refills and they are a fraction of the cost: about $5.00. The one to look for is labeled, "Fisher Pressurized Space Pen Refill - SPR-F". As near as I can tell all the pens use the same, $5.00 refill.
(link: example of refill at online store)
With a bit of heat-shrinkable tubing, a pocket clip taken from a discarded mechanical pencil you can have a cool pen that keeps in your wallet nicely. And since it uses the pressurized, Fisher Space Pen refill, it isn't likely to leak in your pocket.
And, it writes upside down. So, let's get to work. This one will take about five minutes.