Every flat-pedal thread on here will get replies reflecting the monopoly that five-ten (now owned by adidas) seem to have on MTB shoes. These are good but expensive unless you are lucky enough to find some in your size which are on sale, and don’t look hideous.
I currently use DC skate shoes, which are generally stiffer than vans. Vans waffle soles grip fine on flat pedal too (Vans were the go-to shoes for BMX riders in the 80’s, for whom the “flat pedal with pins” that we generally use, was invented for), but they are too flexible for longer rides I find, you can almost feel the shape of the pedal through them after an hour.
I noticed Etnies now do some mtb shoes – it will be good to see some more skate companies bringing out MTB shoes, hopefully bring the prices down a bit?