If you are a US citizen or lawfully admitted permanent resident (and thus legally able to contribute to a campaign in the United States), then you can order a flag from the President's official campaign site.

Just use the "Buy Trump Merch" in the side bar on the right.

edit - added "or lawfully admitted permanent resident" per campaign site information.

If you are a US citizen (and thus legally able to contribute to a campaign in the United States), then you can order a flag from the President's official campaign site. .