Look at Empress Miniatures, they do windows etc.
Linka (plaster moulds for building bits) used to do metal extras like windows.
Poundland & similar cheap shops sometimes have small doll's half houses, worth
recycling their doors & windows. Two of the 'buildings' back to back now are part of
my AWI collection.
A European company (sorry lost their name when my e-mails went bugger up) do a
whole range of D&D bits, including doors & windows.
Might be worth a look at 1/48th scale doll's house companies for spares.