If you have flat feet and feels discomfort (aching, soreness and/or tiredness) after long periods of walking, u can first start by trying those off-the-shelf insoles if you can find them at a reasonable price. No need to spend too much on off-the-shelf ones.

If that doesn't help, it's best to go see a doctor about your feet and they may prescribe custom foot orthotics for you.

I myself wear a pair of custom ones that I place in whichever shoe I wear that day and can walk a full day without feeling any discomfort.