Not sure about flat but I have EEE width. Here are a few brands that I have found:
Crispi - work for me. Have to size up about a half size to get width. Run these with a superfood insole.
Danner - have worn them for 25 years going back tot he army. not the best hiking boot designs but plenty wide for me. I have two pair of combat hikers, a couple of other .mil boots and still two pairs of old school Acadias.
Asolo - hit or miss but I found some designs wide enough. The 520 was my go to hiking boot and then the fugitive.
Merrell - I used to love them but I had to leave them. My last set of sawtooths lost their sole way too soon. I am on my 6th pair of MOABs and this and the last pair are wearing out way too fast. The last straw was a set crosstrainers that failed miserably after a few months. IMO their quality has slipped way down and I was perturbed by my CS experience on the trainers.
KEEN - my new merrell. Wide fit is fine. Need more time to comment on durability.