If you are planning in advance then buy your tickets as soon as you can. They sell out quick. They do release some tickets the day before but they also go quick so check as soon as they are released in the morning. They do allow day of shuttle tickets but that's just from 3pm-5pm and a limited number. If you don't get shuttle tickets there is still a good amount to check out near the visitor center. Or I believe you can even bike the shuttle route.
Britta is correct. They go on sale in the morning so it's best to be ready to hit that purchase button because they sell like hot cakes.
What Britta said it's accurate. You get them online at the parks website and they sell out really quickly! If you miss them, you can still buy some from local travel companies but they cost WAY more.
I didn't use a shuttle so I'm not sure in that.
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