As general entry is free, tickets are not typically required for basic access. For paid events, onsite purchase availability depends on the event organizer.
For standard admission to De Grote of Jacobijnerkerk, no tickets are usually necessary as entry is free. Visitors can simply walk in during opening hours. This directly contrasts with attractions that mandate a purchase for entrance.
If there is a special event, concert, or guided tour that requires a fee, the ability to buy tickets on site will depend on the specific arrangements made by the event organizers. Some events may require advance booking online, while others might offer limited tickets at the door. It's best to check the specific event details if planning to attend a paid program.