Interesting that the heels help, rather than aggravate, posture and knee problems. I would have expected the opposite! That's why I always wear flats.
I found some ballet flats on sale last year and gave them a try, and I liked them so much I also picked up a slightly more expensive brand on Amazon.
Sizing was the big struggle. In men's shoes I wear a 9.5 wide, which is to say feet around 27cmx10cm. The
ballet flats I got at Walmart are size 11W, so I figured that was more than big enough for my big feet. Especially considering that they're a soft stretchy leather. I took them home, tried them on, and they nearly cut my feet in half. MUCH too small! I don't have a shoe stretcher, so I put a tennis ball in each toe. A few days of stretching, aided by some work with a hair dryer at its hottest setting, and now they're wonderfully comfortable although I do end up with a faint red ligature mark across the tops of my feet at the end of the day.
From Amazon I got
these Luoika flats that are a slightly firmer leather, but they go up to 13 extra-wide with the promise that their shoe sizes are "true wide width", which is to say the whole shoe is wider rather than just the upper. Burned by my experience with size 11 I still went with 12WW, and it was a mistake -- wide enough to fit, but at least 2 cm extra space at the heel. Sent them back and went with 10WW. Those fit at the heel without too much extra, but now we're back to the toes being too small even at extra-wide. So once again with the tennis balls and the hair dryer, and now they're rather comfortable. I prefer them for my daily home-office wear, then use the softer leather flats as my lazy lounging slippers.
You can see a [url=ttps://ralphinadress.wordpress.com/ballet-flat-comparison]comparison of my three types of ballet flats[/url] on my Wordpress page.