Dude, go ask women where you can find a nice pair of "flats."
To my knowledge, most men's dress shoes do not have flat soles. And I have a lot of dress shoes.
Oh! Actually, you are likely to find flat soled dress shoes from designers who do NOT usually put out (hah) conservative styling dress shoes. i.e., if you go with one of the traditional dress shoe brands, you won't find flat soles (likely not). Go with a designer who put out some flat soles as part of like a limited collection or something. They tend to not do super conservative designs but still look dressy.
Some of those sailing shoes (I forget the brand) look dressy and have flat soles.
And try using Zappos at least to look for the shoe.