The Cybiko is a handheld computer from the early 2000's. It had an LCD screen, full keyboard and could wirelessly communicate with other nearby Cybikos. There was also an MP3 addon which could take SmartMedia cards. An Xtreme model was released which had more memory and a faster processor. The standard SDK was free so you could create your own games and applications!
They have also been mentioned in an XKCD funny as well: https://xkcd.com/2699/
Given that they have probably been out of production for two decades, your best bet is probably second hand shops and markets.
Online, eBay* is an option but Cybiko prices are a little steep at the time of writing!
* This link goes via TopCashback which gives you up to 10% off at eBay (and discounts at other online shops) and also helps this site too!
Below are links to applications, games and guides create by me for the Cybiko handheld computer!