I would hope you're looking at hydraulic, rather than air bags. 

If you're simply wanting more downforce, this can be done with springs also.  There are 'assist' springs that can be added to the outside of the parallel link arms.  Springs aren't all bad -- they adjust instantaneously, whereas the air bags are extremely slow to compensate (and the older Deere SeedStars are some of the worst, with adjustment times of a mere 1 Hz -- once per second, which by then the planter has traveled ~ 8 ft at 5 mph; it's constantly adjusting to wheel tracks, etc., that aren't predictive at all about what the row unit is encountering next).