You can purchase tickets for e-ride night at guest services at your resort when you check in, then trade the tickets for your bracelet the day of e-ride night after 4pm. There a a few different places in MK where you can trade tickets for bracelets.
You can also buy tickets at MK the day of the e-ride night, although I don't recall exactly where. Guest services, I would guess.
The price is $12.