They are fairly rare. They are the least printed bill [1] and aren't included in a typical cash register, so just randomly stumbling upon one in the wild is a pretty rare. Sometimes people think they are fake because they've never seen one before. The only reliable way to get them is to specially request them from a bank or order them from the government. Also, because of the novelty, people tend to collect them, at least informally, so they don't tend to circulate much either.

[1] https://www.bep.gov/currency/production-figures/annual-produ...