Very VERY rare, honestly. Not as rare as Unstable nodes AFAIK, but I've seen two out of seemingly thousands. One unstable, mind you.
If you want a hungry node, but can't be arsed to find one proper, use creative mode option; grab a node from either creative mode or NEI, and place down nodes until one is hungry.
Or, just do what @Padfoote said and install one of those mods; I suggest at the very least using Advanced Thaumaturgy to make a node hungry(IIRC it can do this). Also, I suggest that @ThatOneSlowking learn how to use the Warding Focus. Obsidian is for chumps, mate. Why not make anything you want as tough as bedrock until you say otherwise?
If you want a hungry node, but can't be arsed to find one proper, use creative mode option; grab a node from either creative mode or NEI, and place down nodes until one is hungry.
Or, just do what @Padfoote said and install one of those mods; I suggest at the very least using Advanced Thaumaturgy to make a node hungry(IIRC it can do this). Also, I suggest that @ThatOneSlowking learn how to use the Warding Focus. Obsidian is for chumps, mate. Why not make anything you want as tough as bedrock until you say otherwise?