If advance tickets are no longer available, be ready to try online at 6am that day. If that doesn't work, be in line by 7.
Your best bet for any museum is to get there early in the morning to get tickets. If you were looking for tours for State Department, White House, FBI building it's best to go through your congress person to get tickets which could take up to 3 months.
Online. And go early
Show up early on that day. Very early. Most museums in DC are on a first come first serve basis. If not, check their website. You may be able to purchase them in advanced.
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