Have a look in your area for a podiatrist who specialises in biomechanics. Often they will take measurements of your feet, leg length, analyse walking and running patterns and perform a pressure plate analysis etc. One near me charges £40 which includes the initial assessment, the orthotics (insoles) and a follow-up appt.
Avoid shop bought orthotics, they are not specifically designed for YOUR feet and are often WAY overpriced. Get the professionals to custom make some for you.
Are you having any problems at the minute or is it more of a prevention thing?