It depends. If you have very minor pes planus, Dr. Scholls or Air Plus may give you some relief. They wear out quickly though (about 4-6 weeks), and really do not provide sufficient support for any significant degree of pronation. Super Feet and similar stores are no better - less supportive in fact - but much more expensive. I suggest you go to someone who can fit you for custom orthotics - they last longer (one to two years) and will provide the support that you need.
This is the company that I use to source my orthotics (I measure and mold in the office):
http://orthodynamics.com/They also have some "off the shelf" orthotics that are quite good.
A casual stroll through the lunatic asylum shows that faith does not prove anything.
Friedrich Nietzsche