Do I need to purchase a ticket to visit the Smithsonian National Air and Space Museum in Washington, DC? I searched online and I couldn't found any online source to buy tickets. Is this a free entrance museum?
The museum is free to enter. The Virginia annex of the Air and Space Museum, the Udvar-Hazy Center, has a parking fee but it also is free to enter. The reason that you cannot find a source for buying admission tickets is that no such tickets exist.
I have visited both museums several times over the past few years and there has never been an entrance fee.
There are security checkpoints to enter the main and annex museums, but no documents are required to pass. They are mostly concerned about checking bags. Sometimes you may be required to walk through a metal detector. Leave the guns at home. The lines for the security checkpoints can get very long during peak visiting times such as weekends in midsummer. Plan accordingly.