If you have a pen that offers multiple ballpoint choices, you're at the right place for new refills. Most brands of multi-pens utilize the same mini ballpoint refill. It measures just a little over 2-1/2" long and is a straight tube with a very small point. Colorado Pen offers a wide range of colors in the Monteverde mini ballpoint refill. This Monteverde refill utilizes liquid ink, the lowest viscosity, for smoother writing. But always remember to retract your ballpoint before pocketing, as this ink will leach into fabric quite readily.
Your multi-pen may also feature a pencil point. The correct lead is determined by diameter and most multi-pens use either .5mm or .7mm lead. If you're also shopping for eraser replacements, those are manufacturer specific and usually require your multi-pen brand to select the correct ones. To view pencil lead and eraser refill options, click on this link: Pencil Lead and Eraser Refills
And for those who would find a pda stylus point useful, Colorado Pen even offers one of those. It can be installed in place of one of the ballpoint tips.