Okay, first of all, we'll need to figure out how much food it really needs. Let's say your dragon is the size of an Amphicoelias fragillimus, a real castle-sized land animal, which is estimated to have weighed 120,000 kilograms. Assuming a reptilian physiology, the dragon would need to consume 1.2 million kilocalories a day. Damn.
That's what the formula coughed up at least; however, I suspect the daily energetic requirement would be lower, since it's the same as a blue whale's, which is both heavier and an endothermic mammals. Let's call it a million.
To make this more plausible, I recommend the following planetary conditions:
* High atmospheric oxygen levels, ideally about 30%
* A dense atmosphere
* High global energy availability to maintain many-leveled food chains with broad bases
Low gravity isn't very important, since it makes the dense atmosphere less likely, which you need more. Now, to properly answer your question:
First idea; aeroplankton. Earth has aeroplankton in real life - viruses, bacteria, fungi, protists, algae, spores, pollen and seeds, all carried on the wind. However, there's not that much of it, it's not very dense, and a lot of it is too high up to be harvested by your dragon. Given a dense atmosphere, it's plausible if not likely that these problems wouldn't be present.
Alternatively, you could take inspiration from the aforementioned Amphicoelias, which was a "veggiesaur". It was essentially the size of a castle, as you can see above, and it got by eating just plants. Specifically, it stripped the branches and bark of large trees, probably clearing whole forests like modern elephants.
Given that sauropods of such size could've existed, castle-size, even on land isn't that far fetched. However, I doubt your dragon could fly except in a very dense atmosphere. You could decrease its caloric needs by giving it an even slower metabolism (which would make it sluggish) or having it hibernate for long periods.