Insoles to treat flat feet
FLAT FEET force your foot to hyperpronate, which mean that your leg will bend inward and your knee will not be in a straight position, but rather in a inward position.
This causes a lateral pull on the kneecaps, which track improperly in the femoral groove. As a side effect the vastus lateralis (lateral quadriceps) and iliotibial band work more than the vastus medialis (medial quadriceps). Hence the latter atrophies and the first becomes too strong and even develops trigger points, tightness and tendinitis in the insertion point with the tibia.
A simple way to help reestablish a balance is to support the metatarsal arch with an insole. This should be calibrated - custom made by an podiatrist.
If you want an adjustable and cheap insole, you can buy a commercial insole and stuff under it a fragment of sock (a pair of scissors and an old sock will do). You can adjust the position of the sock under the insole until you find the most comfortable adjustment
Of course this will not be a cure-all, but every little bit helps.