Is it obligatory to buy another sheep for the 'aqiqah if the first one died and the financial situation is bad, or is the intention for the first sheep sufficient?

Aqiqah is a Sunnah, not an obligation. If someone buys an aqiqah animal and it dies before being slaughtered, and he is unable to purchase a replacement, it is hoped that he will be rewarded for the aqiqah. The Sunnah of aqiqah is not invalidated by the death of the sheep. Whenever the father is able to perform the aqiqah for his child, even if it is close to the age of taklif (accountability), it is recommended for him to do so. If the child reaches adulthood, he performs the aqiqah for himself.