The USB cable is just any old regular A to mini-B USB cable. Find it anywhere you can get USB cables - Radio Shack, 7-11, lying in the street, etc.
The AC adapter is another story. You need something that is putting out 9.0 volts DC at 800 mAh, positive tip. You can probably find that at RS, yes. So long as whatever you get has the proper barrel connector to fit in the HP1's power socket, and you meet the above electrical requirements, you should be good to go.