Durva is perhaps the most essential ingredient. These are blades of a specific type of grass.
Lord Ganesh loves red hibiscus flowers. Get some of these along with other flowers like Mogra, Champa, Swastik etc
Haldi, Kumkum are essential. Chandan, Gulal and Bukka are also offered to Ganapati
11 Modaks are offered as prasad. Modaks can be fried or steamed. If you cannot get modak, prepare any other sweet and offer that as prasad to Ganapati Bappa