I also have flat feet and they gave me plenty of trouble over the years, especially in the Army (82nd Airborne Infantry Ft Bragg). After years of trying different insoles I found these:
Sof Sole : The Sole of Performance
They used to be called stable trac but they might just be called stability now. They are made by Sof Sole and can be found at many sporting good stores (don't try foot locker cause they don't sell them). They cost $30 beans and I love them so much I bought two additional pairs, the origional pair is still holding up after about 2 years. Another good insole:
Sof Sole : The Sole of Performance
is also made by Sof Sole called graphite. I used these for years and have three pair. They work well but are more expensive than the stability and they begin to squeak after a while. In fact it is very anoying and is the reason I tried the stability insole in the first place. Hope this helps.
Respectfully
John