No, you cannot buy tickets on site as there is no admission fee for Saint Den Helder Park; access is free for all visitors.
Saint Den Helder Park is a public nature and recreational area, which means there is no entrance fee whatsoever. Consequently, there are no ticket booths or facilities to purchase tickets on site. Visitors can simply enter the park at any of its access points and begin their exploration without any financial transaction.
This free access policy is common for many public parks and nature reserves in the Netherlands, aimed at promoting public health, recreation, and environmental appreciation. Any services within or around the park, such as bicycle rentals or guided tours (if available from third parties), would be separate arrangements not related to park entry. Therefore, visitors do not need to concern themselves with ticket purchases when planning a visit to Saint Den Helder Park.