People mostly look for bok choy substitutes due to its high price and unavailability. So, here is a list of nutritious and pocket-friendly bok choy substitutes that are easily available.
Bok Choy Substitute
The best bok choy substitutes include napa cabbage, swiss chard, yu choy, spinach, cabbage, celery, collard greens, beet greens, tatsoi, asparagus, and radicchio.  
This is the best bok choy substitute provided you do not boil it. Stir-frying napa cabbage will help you retain its crispiness like bok choy.
The color of Swiss chard is easily detectable but other than that it is quite similar to bok choy in terms of nutrition. Make sure you do not cook the leaves and stems together.
Yu choy (choy sum) can be used as an alternative to bok choy. When substituting bok choy with yu choy do remember to add a few drops of lemon to kill its mild bitter taste. Additionally, it also acts as a taste enhancer.
The highly fibrous leaves of spinach are the cheapest alternative to bok choy, provided you ignore its pungent smell. It also differs from bok choy in terms of crispiness. Therefore, as a bok choy alternative, add spinach along with sturdier vegetable like broccoli.
Cabbage is a perfect substitute for bok choy as it is low in calories, high in nutritional value, and is budget-friendly. Combine cooked and shredded cabbage to give the food the crispiness of bok choy.
Although not the best substitute of bok choy, some people do use a handful of celery leaves.