When I did this, I was not staying onsite, and I got the tickets through the meeting I was attending. I took the offsite hotel's shuttle to get there as I did not have a car.
I just came back from a convention at the Dolphin hotel. It was fantastic! A perfect location. I was lucky to stay at the Dolphin. The convention room block sold out quickly.
I got the after 2 ticket for my first day and an after 4 for the other two. I got them by calling the convention services line after talking to someone in hotel registration. These tickets are linkable to your MDE account to book fast passes. It was great having 3 fast passes going into crowded parks in the evening.
The price to upgrade the after 2 or 4 pass to a park hopper seamed unreasonable. It was twice the price ($67 after 4, $72 after 2, I think).
You can get them through the convention. I don't know how many they will allow each attendee to purchase for family members. Might be up to the convention planners.