After much searching, I'm finally finding a few options for gothic windows. Of course, a lot seem to be out of stock at the moment but since I haven't decided on a dollhouse design yet that shouldn't matter. One tip is to use the word church in your searches.
The 2 on the lefthand-side are from Wood Things and are the best priced of the bunch. Middle left are from Earth and Tree. Middle right can be found on 2 UK sites, York Miniature Scene or Baccraft Miniatures. On the right, some great windows On Being a Mini Mum found on ebay, which I sadly haven't seen on sale. Guess I could message the seller and see if they still sell them.
I also like these mullioned windows from Maple Street Dolls Houses. Hogwarts had a mix of windows but with a small dollhouse I don't want things to get too messy if I mix and match styles. I suppose if there's diamond leaded panes in two window types that could tie them together...