From http://www.tortoisetrust.org/articles/plantfoods.html
• Dandelion (Taraxacum officianale)
• Hawkbits (Leontodon spp.)
• Sowthistles (Sonchus spp.)
• Hawkweeds (Pictis spp.)
• Hawkbeards (Crepis spp.)
• Plantains (Plantago spp.)
• Clovers (Trifolium spp.)
• Honeysuckle (Lonicera periclymenum) [preferred by T. hermanni]
• Cat's ears (Hypochoeris spp.)
• Vetches (Vicina spp.)
• Trefoils (Lotus spp.)
• Mallows (Malva spp.)
• Bindweeds (Calystegia spp.)
• Sedums (Sedum spp.)
• Ivy-leaved Toadflax (Cymbalaria muralis)
• Also (observed eaten by T. hermanni and T. marginata in Greece):
• Robinia (pseudo-acacia) leaves
• Wild clematis
• Acanthus
• Nettles

• Dandelion (Taraxacum officinale) leaves, stems and flowers.
• Red clover (Trifolium pratence) leaves, stems & flowers
• White clover (Trifolium repens) leaves, stems and flowers.
• Greater Plantain (Plantago media)
• Ribgrass or Ribwart Plantian (Plantago lancealata)
• Smooth Sow Thistle (Sonchus ol¬eraceus) leaves, stems & flowers
• Prickly Sow Thistle (Sonchus as¬per) coarsely or finely chopped.
• White-Dead Nettle * (Lamilim al¬bum)
• Red-Dead Nettle (Lamium pupureum)
• Chickweed * (Stelaria media)
• Smooth hawks-beard (Crepis cap¬illoris) leaves & flower
• Hedge mustard (Sisymbrium offl¬cinale) young plants
• Bramble (Rubus fruticosus) shoots, tender leaves & fruit.
I also think you can feed them the following supermarket food

Chicory (dandelion greens) and Endive

DO NOT feed lettuce, spinach or cabbage.