That’s such a good question! I love xorta and they’re soo good for you. Technically, there are two popular types of Greek xorta (vlita and radikia). Radikia are also known as dandelion greens are much easier to find oustide of Greece. You might have to go to a local farmers market and ask around.
You can technically use any greens that you find in the store that are able to be boiled, like collard greens, mustard greens, spinach, escarole, etc. It being Greek is more about the seasoning - it's usually lemon juice, olive oil, and sea salt. The greens themselves are less important. You can also forage for your own. Vlita is amaranth, you can use dandelion greens, and another great one is "levethies", also known as lamb's quarters or wild spinach. In Greece, they use whatever is available.