Well the ComboBox is simple. There is no JTable in J2ME, now there is a product that is open source and free called J2ME Polish that can make some prettier screens and emulate a JTable.
As far as contacting a database to fill in your ComboBox, you must consider airtime. This costs the user money using up their airtime. So I would suggest either storing the data in the recordstore on the device or put the values in your code or resource. If you make the phone contact your database each time you run the application, then you are using up people's money and airtime.