The only time there is an opportunity to get tickets in advance is on the 1st Wednesday of the month. However, if you go to the website you will see that the up coming advance tickets will be for visits in May.
You have to call. The wait can be up to 2 hours sometimes.
Every time I've been I was able to just walk right up and they had staff with extra tickets near the entrance
Check the website at 6:30 AM the day of your visit to check for tickets.
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