I presume you are asking about the chumby one. The charger tip required is a 0.9mm I.D. x 3.2mm O.D. tip. It's not terribly common but they do exist in some universal adapter kits. 0.8mm I.D. tips (same O.D.) work as well, and they have a snugger fit. Make sure your charger has the appropriate rating, though -- chumby one is rated for up to 2A at 5V +/-5% -- typically we consume much less but there are configurations where we can get close to that amount of power consumed, particularly if you have a power-hungry USB peripheral plugged in.

And of course, the official policy of the company is to recommend you only use the adapter provided with the chumby One -- the warranty may be voided by using your own power adapter, as you will damage your device if you put in the wrong kind of adapter with the wrong rating!