Is it permissible to buy rice for Zakat al-Fitr now and distribute it at its designated time, fearing a rise in prices and the inability of some to purchase it later?

There is no harm in a Muslim purchasing and storing goods they need before prices rise, for their own necessity. Then, when the time comes, they should pay Zakat al-Fitr from these goods, based on what was narrated from Umar (may Allah be pleased with him), that the Prophet (peace be upon him) would set aside for his family enough provisions for a year. This indicates the permissibility of storing provisions for one's family for a year.