If they are not passing them out when you arrive, they will probably send you to guest services in the City Hall to get it. When DH & I attended the party we arrived early and they were already giving them out to people who had their tickets right at the entrance (actually it was before we even went through the area to have our park pass scanned).
Partygoers don't go through the same turnstiles as the other guests. Beginning at 4pm there is a separate set of turnstiles opened just for party attendees. On the other side of those turnstiles are CMs waiting to place wristbands on the guests.