windows won't stay open

Yes, most use tension springs. Depending on how old the windows are, you should still be able to find parts.

On the side of the frames connected to the sash, you should find a tube, disconnect this at the top and open the window. You should be able to pull it out from the bottom. Contact several glass replacement comapnies and see if they carry the springs.

Now to put them back, UGH this is the bad part.....slide the spring in place, connect the top. now twist the spring to tighten ( spend the money to buy the special tool) and clip it to the bottom of the window.

Most springs are similar, the method of attachment to the bottom of the sash differs.