I was just talking to Keith at Redlands about air bag replacement. He says he removes them and takes them to his truck part supply place to match. If its in a big town, should be an off the shelf item, and ~$125 ea. Both fronts & rears should be not very challenging, but use jack stands for safety.
Call a truck stop or truck repair place near you and ask, "where would I go around here for replacement air bags?"
Also, I believe, the part#'s are on the metal plate lid. There may be a choice of with & without rubber bumper inside; you need the one w/the rubber bumper.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project