I purchased them online for the special event. If there is no special event I think you can go and purchase an admission ticket any day you want to go. Have fun. The mansion and the grounds are just beautiful !!!
It is encouraged to purchase in advance, however I believe if they are not sold out capacity wise for admission you could pay when you get there. Call ahead just in case I am wrong. Their gift shop is currently online only. Events require advance purchase.
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