You will find them on a menu in most traditional Greek taverns- as they are a staple in the Mediterranean ... You can use endive, chicory, dandelion (Radikia), wild spinach, purslane, beetroot leaves, ...

a Greek Horta — which is greens like dandelion, amaranth, and chicory, boiled and dressed with olive oil — or Jamaican Calloloo, greens cooked with onion, garlic and sometimes coconut milk ...