You can also try R Nicholls.
http://www.rnicholls.com/en/index.php. Like you I also am flat footed and have very wide feet. You should get yourself properly measured footwise. Standard width for mens shoes/boots is "D" then it goes to "E", "2E", "3E", "4E". Most of the time "Wide" is "E" and is usually still to small for me. Im a "4E", its pretty damn hard to find a good variety in those sizes if not impossible. Also be prepared to pay a lot. I hate shoe shopping, so I buy for comfort and fit and the most durable I can find so I can put it off as long as I can, which means around $150-200+ range.
If you go to Nicholls I recommend the Magnum's, I've had mine for 1-2 years and they have yet to let me down.