Is it permissible for me to use my cousins' money to buy food for myself without their permission, knowing that they would agree 90% of the time?

There is no harm for a Muslim to take a small amount of money from his Muslim brother, of what people commonly overlook, and provided that the taker knows his brother is perfectly fine with it. For customary permission takes the place of actual permission, and this has been indicated by texts from the Qur'an and Sunnah, among them His (Allah's) saying: (Nor [is there blame] upon yourselves to eat from your [own] houses or the houses of your fathers or the houses of your mothers or the houses of your brothers or the houses of your sisters or the houses of your paternal uncles or the houses of your paternal aunts or the houses of your maternal uncles or the houses of your maternal aunts or [from houses] of which you have been given keys or [from the house] of your friend). As for one who takes something without reasonably assuming the owner's consent, it is not permissible for him to do so, due to the Prophet's (peace be upon him) saying: (The property of a Muslim person is not lawful [to take] except with his willing consent).