After weeks of shortages across the country, Americans are gradually finding it easier to get their hands on at-home rapid antigen tests. That’s in large part because the Biden Administration’s massive buying spree is giving test companies the confidence to manufacture and distribute a product that, like the course of the pandemic itself, has an uncertain future.

Executives at several major test companies say that, while their production lines are ramping up, they are prioritizing orders from the federal government in order to deliver on those lucrative contracts. Because of that strategy, some other buyers—including retailers and state governments—are still finding it challenging to get enough tests to meet current demand. But supply constraints are expect…