Praise be to Allaah.
If a woman buys men’s clothes to wear them, this is a kind of imitation of men, if these clothes are distinctive and are usually worn by men, such as the regular thobe (“galabiyah” or “dishdasha”) or shirts. But if it is a garment that is worn by both men and women, there is nothing wrong with that at all, such as sleepwear and other garments that are worn by both men and women.
In general, I would say that everything that is only for men is not permissible for women to wear, and everything that is only for women is not permissible for men to wear.
But if it is a garment that is common to both sexes, there is nothing wrong with that, or if a woman is forced by necessity, such as in some Muslim countries where there is extreme poverty – if a woman finds a garment that is not see-through and does not show the shape of her body, and covers her properly, and it is regarded as a man's garment in some Muslim countries, there is nothing wrong with her wearing it because of necessity and the need to cover the ‘awrah, because this is a case of necessity, not ordinary circumstances.