I am trying to purchase tickets online. I only see reserved seating. Do I have to pick my seats or can I just purchase tickets and find a seat when we get there?

You are going to have to pick your seats . That's how they give you your tickets.

You do have to reserve your seats in advance- that's becoming the standard across the board. The seating is arranged just like concert or event seating, with A being the front and building out from there. The positives outweigh the negatives, especially since you're buying online. It means you can pick your seats in advance, and take your time getting there, no worries about getting there early to get the seats you want (unless you didn't get them online) they're already reserved for you.