PDAs are largely a dying breed, with smartphones (combination PDA/cell phone) taking over the market share. HP is currently the only major manufacturer producing PDAs in the US, with retailers such as Best Buy frequently having store models you can try out.
Alternatively, an unlocked smartphone eliminates the need for a contract and can be used as a PDA, ignoring the cellular aspect. If you go that route, check out the in-store displays of electronics stores, cellular providers, and even Walmart. If you like what you see, you can buy an unlocked model from select online retailers, such as Newegg.com, or from reputable sellers on Ebay, usually with the full manufacturer's warranty intact.
As Bob said, though, a slew of lightweight, lower-cost, touchscreen tablets are going to appear later this year, so if time is not of the essence you may wish to hold off and see what selections are available this summer/fall.