I may not be the best source because I’ve only done it once, but I’ve never known for them to sell out. I usually scope out the stand on the way out because it’s one of the hidden gems I learned about here a few years ago and have passed along to friends.
Also, somone correct me if this is wrong, but I thought you can purchase the discounted Express before 11pm so you can start using it right at 11. I wouldn’t try to buy it when you arrive for the night, but if you pop by a kiosk at like 10:30 it’s worth a shot to ask.