There are two Guest Services locations at Downtown Disney. One is next to Team Mickey and the other is near Planet Hollywood.
At MK, there are Guest Service locations at the Ticket & Transportation Center and at the main entrance of the park.
When we got are new ones last September, the T&TC GS location was not open. We didn't go to the MK until late afternoon, so I don't know if it was closed due to the time of day.