Flat feet pain after standing for longer periods of time?

Custom orthotics to correct for over pronation of the foot

You probably have Plantar fasciitis which is a repetitive strain injury. Buy some sets of insoles: 'Sorbothane CushNStep' work well for me and also wick away moisture. I replace them about every 6 months.

I use sole inserts. You have to experiment with them - you'll like some more than others, but they all have a relatively short life. I've been to podiatrists, who have never done anything for me.

Change your insoles and do so regularly.  I do contract work and when I'm on the job, I'm working 7-12s, much of which is on my feet, for months at a time.  I change my insoles in my work boots at least once a week.  It makes a big difference.

Yes you can buy orthopaedic insoles.
Try google.

I was caught with the same thing. My pain was excruciating. One thing many people don't realise, is that when you're standing/moving around, your feet swell. If your shoes haven't enough room to accommodate this swelling, you get painful feet. The longer you do this, the worse the pain will get. I don't know if you have a 'shoe' policy where you work (like I did), but I spoke to my manager about it. Many athletic shoe shops specialise in shoes with built in arch supports. Of course, if it's for work (especially if there is a shoe policy) don't get a 'colourful' pair that many have. I got a black pair that didn't stand out, but did the trick. And the manger was happy.