Petsmart or Petco should carry it (ours do). With dry you have to do it slowly - put a small handful of the new in the bowl and mix with the food he's eating now, then over 2-3 weeks times gradually increase the new food and decrease the old.
I havent seen it in petco or petsmart either. Go to the chicken soup pet food website. They will have a link so you can search for suppliers near you. I have my cats on the Adult light formula and they like it.