Check out Walthers Trains. They are a big wholesaler of model RR equipment. A few weeks ago I scooped up a bunch of windows in O scale (1/48th). Maybe their selection in 1/87th might work for you. Scratching windows is a REAL drag !

another possibility might be the shipbuilder suppliers

https://www.cornwallmodelboats.co.uk/cgi-bin/sh000001.pl?WD=windows&PN=44105%2dEndeavour%2dLaser%2dEtched%2dWindows%2dWood%2d44105%2ehtml#SID=1558

http://frenchmanriver.com/search.asp?keyword=window+castings&search=

you could also buy a kit, rob parts you need, and use the remainder as building stock, but it might not be the most economical way. time for acquisition & style needed value may outweigh $$$ cost - only you can make that decision.

Can't remember who it is, but here in the UK there was a company selling windows & doors for OO gauge (1:76 scale) UK/European outline stuff. As Southpier says, don't get too hung up on exact scale!

I think because you are modeling 1:72 scale, choices might be limited.

since doors & windows come in such a wide variety of 1:1 sizes, I would not get too hung up on the difference between 1:72 & 1:87. the casings might want to be narrowed a but to suit the style which you are representing, but other than that I cannot see the drawback:

https://www.tichytraingroup.com/Shop/tabid/91/c/ho_framed-windows/Default.aspx


https://www.modelscenerysupplies.co.uk/scratch-build/model-windows-doors-guttering


https://www.micromark.com/200-piece-HO-Scale-Architectural-Parts-Assortment

https://www.walthers.com/search/category/products/accessories/scale/ho-scale?q=window

I'm building a 1/72nd scale diorama of a WWII railroad station and so I would like to know since I will have to scratch build the building/shack,I would like to know who makes WWII windows/frames and doors in the 1939-40's or where can I get these items?
If no one does make the window frames and doors,I will have to scratch build them.
Any information will be of great help.
Thank you.