I don't like to give recommendations, but if you search the forums, you'll see that a lot of people, including me, use the Freedom Keychain 2000. It's available from the CrackBerry store. There are other models available. To use the Freedom model with Google Maps, you also have to run BBMaps. Google Maps doesn't provide a "Start GPS" command.
Posted from my CrackBerry at wapforums.crackberry.com