AK: Guest Relations window outside the main entrance (to the left, as you're looking at the line of turnstyles) or Guest Relations lobby inside the entrance(just past the line of turnstyles, on the left hand side)
Epcot: Guest Relations window outside the turnstyles at the far right hand end, or Guest Relations lobby inside, just past Spaceship Earth on the left hand side.
MGM: Guest Relations window outside the turnstyles on the left hand side near the kennel, or Guest Relations lobby inside the turnstyles on the left hand side.
MK: Guest Relations Window outside the turnstyles at the far right hand end, or City Hall to the left as you come out from under the Train Station.
Downtown Disney: Either of the 2 guest relations locations. (I'm not as familiar with DTD as I am with the 4 main parks, I can't tell you exactly where the GR locations are)
Note: There are 4 designs of birthday buttons: 1) (circular) Castle Button - issued by MK and MGM --- 2) (circular) Tree of Life Button - issued by AK --- 3) (rectangular) "International Greeting" button - issued by Epcot --- 4) (rectangular) Downtown Disney button - issued by DTD