Most grants for MBA students are provided through the school itself, however there are outside source for grants such as the federal government or private organizations. Note that many schools or organizations refer to MBA Grants as MBA Fellowships.
Contact your finacial aid office as early as possible to inquire about any grants for MBA students. Schools tend to have a limited amount of money for grants, so getting requests or applications in early can be the difference between getting a grant or not getting one. Also, make sure that you have filled out and submitted the FAFSA form, since some grants or fellowships are based on financial need.