Definitely Korshak. If the salesman Gary Murray still works there, I recommend him. Hard to say whether the flagship, downtown Neiman's or the Northpark store would be better, if you had to choose one. I would go with the downtown one but that's debatable. While downtown you may want to see the Art Museum and the Kennedy Museum at the Texas School Book Depository. Along with the others mentioned, I used to hear good things about Pockets, but never went there. While you're in Highland Park Village you can always go to the barber shop there, which is not expensive for a haircut, shine, and manicure. That's where the late Stanley Marcus always went. See www.chowhound.com
if you need food recommendations. I'm sure the Mansion would be fun for dining. It's also a great hotel. Enjoy. Culwell & Son, also mentioned already, is across the street from Southern Methodist University. Shopping there gives you a slice of conservative Dallas white male life. Sack and other traditional suits, a barber/manicure/shine place, a formal wear shop, a sport clothing shop, a boys shop, and a cleaners, all under one roof. For lunch there's a German deli/restaurant/sausage shop called Kuby's in the nearby Snider Plaza shopping area.