Second that - go to a shop where your gait will be assessed by expert runners, as there is more to it than just shock absorbing.
If you want to avoid big chains, I have been going for a few years to Run and Become near Victoria. No fancy gait analysis machine, but will they take as much time as needed to watch you running around the block and advise you. They never got it wrong with me (and no, I am in no way related to them, just a happy customer).
In any case, happy running!