Lowering Kits - Drop Kits from Belltech, DJM, Ground Force & more
Here is a link that offers Lowering Kits for your Application...Or you can always Google it or check through Ebay.
As for how much to lower it, I'd Check Out www.Cardomain.com and search for the range of Years/Make/Model of that vehicle to see what other Suburban owners have dropped their SUVs down to.
This way you can get a visual and possibly contact the owners to ask their opinions of the kits they used and where to purchase them.