They get it from the part of the plant that DOES have chloroplasts. The green part of the onion makes glucose by photosynthesis and sends it to the bulb.
I'm not sure that there are plants that do not have any chloroplasts, however I do know there are many different ways for plants to get food.
Some plants are parasitic, like mistletoe, and grow using nutrients from other plants. Others like the Venus fly trap and the sun dew are carnivorous and trap bugs and small animals and digest them using enzymes for nutrients.