Perhaps check the New Balance website. But, let me tell you, I have REALLY flat feet... To the point that it becomes painful to walk even 9 flat holes... I added Lynco L725 'Golf' orthotics to my golf shoes (remove the stock padding first) and I am walking 9 again. You can also use the L425 'sport' model, it's a little more cushiony.
I swear by these orthotics but they are about $60/pair retail (look on the bay for a deal). I wear them for work, golf, sneakers, everything. They make a big difference, IMO.
Here is their website (I hope this ok per the rules, forgive me if not)
http://www.aetrex.com/html/lynco.shtml#sportsThe plus for you is that you can keep your existing footwear. Hope this helps :-)