regarding the expression "buy foreign"

I came across the expression "buy foreign" which means buy foreign product(s), as I was studying "verb+adjective" phrases such as stay slim, stay healthy, test positive, and etc. (In the book I have, "buy foreign" was listed in the same category of "verb+adjective" with above phrases: stay slim, test positive, ...

How come it is possible for the phrase not to include a noun such as product or something like that? On the dictionary, there was no "buy domestic" expression and it was always used with product like "buy domestic product."

How should I understand this? Is it just one of fixed expressions most people use? or Is there any relation or the way I can infer the usage of "buy foreign" (meaning buy foreign product) in the context of "verb+adjective"? I know the adjectives in stay slim, test positive describes and supplements the state of subject as subjective complement, but I cannot understand how "buy foreign" is realized.

The expression "buy foreign" was in the sentence that follows: The Japanese are being encouraged to work less, play more, save less, spend more and, while they are at it, buy foreign.

I'd very much appreciate if you could give me any help.
Thank you.