The AAMC provides a Fee Assistance Program to those who qualify for financial assistance for the MCAT and AMCAS. The timeline for this can be longer than you might expect so be sure to read over their website and plan ahead.
In addition, Brown University's Curricular and Co-curricular Funding is available to assist undergraduate students in supplementing expenses for standardized test preparation costs, including the MCAT. To apply, log into UFunds, search for the "Emergency Funds, Curricular and Co-curricular Gap (E-Gap) Funds" grant, then apply through the "Professional Development, Curricular, and Co-curricular Assistance Grant" section. Please note that it is not necessary to list a recommender for this application.