You will have many opportunities to purchase souvenirs in Arusha and while on safari.
The souvenir market with the best prices is located in the center of Arusha town, about 500 meters southwest of the central post office/clock tower on Sinoni Road. You will have to bargain at this market; you should pay about 50% of the asking price.
There are also many opportunities to purchase souvenirs while on safari. Most hotels have souvenir shops, and there are a couple of large souvenir shops along the way: Oldonyo Orok and Cultural Heritage (expensive).