I have incredibly flat feet too. That, in combination with really narrow heels makes it really hard for me to find normal shoes or workout shoes that fit.
The only insoles I've ever (successfully) worn are Spencos. I often have to trim off some of the arch support in those, though. Sometimes I buy them when I buy a new pair of shoes and put them in right away. Have you ever noticed how pitiful the insoles that come in some pairs of workout shoes are? I wonder why they even bother sometimes. Right now, I'm wearing Mizuno Wave Alchemy 2001's for running and Ryka XT-ETS's for aerobics. I'm not totally crazy about the Ryka's I have right now, but every other model I've had from them has been great.
The Mizuno's, on the other hand, I am absolutely smitten with. I've always had Saucony's, Brooks or Asics for running, but the guy at the running store recommended these. They fit my feet incredibly well and they weigh absolutely nothing . I plan to buy these forever.