I haven't specifically seen them in any stores, but I would say try somewhere like Prada, Gucci or D&G, or for more affordable prices try Next or Zara.
A shop that sells clothing for gay males/females?
I've seen a few shirts like that on ASOS.
or Hugo Boss/ Ted Baker
I've been to Topman today they didn't have these types of shirts.
I seen them everywhere if you're in Central London - Oxford street.
Burton has them, Topman might have them I've seen a few and River island have them also. Burton does white collar with a black/grey/white line going down but the rest of the shirt is one colour.
Whoever says this is "gay" doesn't have fashion sense, you could make them look good if you know what to wear with them.
Try ASOS, if not try all mens clothes shops basically. I would suggest Topman, Burton, Blue Inc, perhaps?