Yes, as far as I know can buy at any point in the night. In fact, best bet is to get it far enough before 10 that you can be at your starting Express house at 9:59 ready to go (I'd still have a game plan in mind to knock out what you can).Can you get the "after 10pm" express pass when the event just starts?
And can you get them at any random food or store? Or are their specified spots to obtain them?
I've always bought it at one of the "street vendors." But I imagine anyplace selling Express sells it. They're pretty easy to see once you're in the event, plenty of signage.