Theory on how it's even possible that Harry can get his clothes 2nd hand.
Harry is 6.9 , and now brimming with muscle. The idea that he can just find salvation army clothes that can fit him is probably the most unrealistic thing in these books.
I see 2 possible explanations off the top of my head for how he's able to find these clothes
He has a resizing spell that we just haven't seen.
His fairy godmother is shopping for clothes that will fit him and strategic places tailored clothes where Harry can find them.