If your tortoise can just graze outside, it will find good food on its own. Grass and weeds are what it eats in the wild.
If you have to buy something, do NOT buy alfalfa. DO buy Timothy hay.
The best is to give a variety of high fiber material.
You can grow things for your tortoise. Some people do that in kitty litter boxes in the house. Other people just let the tortoise graze outside like it would in the wild.