According to Matt Fitzgerald, "Your respiratory muscles may also become tired. And to the degree that these muscles fatigue first, it is their fatigue—not that of your legs—that limits your performance. In fact, as your respiratory muscles begin to fatigue, your nervous system will redirect oxygen from the muscles of your limbs to those of your diaphragm to keep them going. Thus, during running your legs may fatigue because your respiratory muscles have begun to fatigue first—and to prevent these muscles from fatiguing to a dangerous extent." -How Fit Are Your Breathing Muscles
Power-
Lung is probably the industry leader, although I did find a few less expensive options. I believe there may be something to this- purely subjective but I believe swim intervals helped my breathing- of course those also involve high heart rate, anaerobic metabolism, exhausted muscles, and the rest.
