You can buy them here. You can also buy them online from Statue Cruises. Make sure the source is official, there's loads of scam cruises that don't stop at the island. The real one does both Ellis Island and the statue. There are three levels of pass, get into the island, climb to the balcony (worth it, includes a museum) and crown. If you're into engineering or architecture the crown pass is worth it. Otherwise you can't see much.
You can buy tickets at harbour and online as well. https://www.statuecruises.com You can search for other sites for better pricing.
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