Other popular substitutes are rapini, dandelion greens, or beet leaves. Anything green and leafy, basically.
The recipe couldn't be simpler.
You'll need:
1 large bunch of vlina, or other greens
a splash of vinegar
salt
lemon juice
olive oil
a clove of garlic, thinly sliced (optional)
Pull the leaves and smaller stems off of the bigger pieces of stems (I'm not a huge fan of those parts, you can chop them up and use them if you want), and put the leaves into a large bowl filled with cold water and a splash of vinegar. Let them sit, and if necessary change up the water until the greens are clean.
Boil some water in a large pot with a good amount of salt (2-3 tsp). Add in the greens to the boiling water, and cook for 15-20 minutes, or until they are good and wilted.
Drain the water, add a few good splashes of olive oil, the garlic (optional - this is my own personal taste) and a big splash of lemon juice. Taste, and add salt if you think it needs it.
This is great as a side dish, or even as a lunch with some tzatziki and bread. Yum!
Have a great day!