Yes, at each park's main entrance other than MK, it will be one of the ticket booths, clearly marked "Will Call", which is after the bag check. At Epcot, there is also the International Gateway over by the Beach Club, and it will be one of the the two booths there, before the bag check.
At the MK itself, where they don't have ticket booths (since those are located at the TTC), the Guest Relations window just to the right of the turnstiles is usually the will call window as well, although sometimes it is in the small round building nearby there - there will be a sign with an arrow pointing to which one.
There is also a Will Call at the TTC, although I bet few actually go there for the GAD exchanges.