Go to the diet section and look for the Plant ID section. There is a great thread there showing all sorts weeds. There are literally dozens of different types of weeds that are good for them. You could also think about growing your own stuff. I got a Testudo seed mix from Tyler and Sarah at tortoisesupply.com and I love it. I also grow alfalfa, clover, gazanias, African hibiscus, pansies, grape leaves, mulberry leaves, squash plants and a whole bunch of other stuff too.
From the grocery store, I favor endive and escarole. I like to add some cilantro, carrot tops, watercress, and collard greens too.
Excerpted from Marginated Tortoise Care Sheet By Peter Watson:
Diet
In the wild Testudo Marginata have a very varied diet, consuming many different weeds and wild flowers everyday. It is fairly easy to replicate such a diet in a captive situation with a little research into edible weeds and plants . Some weeds and flowers which can be included in a Marginated tortoises diet are: Dandelion, Bitter cress, Sow Thistle, Vetch, Clover, Dead nettle, Plantain Evening primrose, Hibiscus, Lavatera and Pansies. This is only a brief summary, there are many other weeds and flowers that can be included, as previously mentioned a little research to familiarize yourself with the various edible plants is all that is needed to create a healthy balanced diet. Marginated tortoises are well known for being partial to snails. I have observed them actively hunting for snails when outdoors. I never supplemented the diet by feeding them snails, but if they chose to find them I allow them to eat them.