depending on what you want there are many companys to go with
here are the questions you need to ask yourself
1: what do i want it to do
2: how fast do I want it
3: how much do i want to spend
after you answer those then you start looking. I know i can get a fast bag kit through my company for a litle over a grand. that is front back and side to side
start here
air ride tech
then go here
other site