Here is a short list of organizations where schools and other non-profits can apply for gardening grants. I would suggest checking locally first. You might find your local Optimist Club would be excited to help fund a Tower Garden in your classroom.
Annies.com. – Donations to schools and other educational programs that help build school gardens to help connect kids to real food.
Whole Kids Food Foundation – Whole Kids Foundation supports schools with a variety of programs and educational materials, including:
Kids Gardening Youth Gardening Grants