The state or federal Department of Education, private foundations, and even the school itself can provide grants for private school students at any level. Depending on the organization, you may need to apply online or through the mail.
Grants are often awarded based on merit, financial need or hardship. If you are looking for a grant to cover the cost of a private school education, the first place to look is your state Department of Education. According to grantspace.org, some state Departments of Education may provide financial aid in the form of vouchers for students in need in primary and secondary schools. The federal Department of Education also determines the need for grants based on a Free Application For Student Aid, or FAFSA, a form that can be filled out online or mailed. These awards are generally for college students.
Private foundations are a good source of grant funds as well. The Foundation Center has a reputable database of grant opportunities that is subscription-based. Houghton-Mifflin Harcourt also has a free database that is very comprehensive. Schools themselves may also have grant programs for students in need. Financial Aid counselors at the school you have applied to are also often able to provide you with some grant assistance.