As ehall says, you can do it either way, but chances are that if you can find a good piece of glass, you'll have to buy the frame anyway. When shopping for it, check very carefully that the pivot pin is not loose in the metal frame of the glass. It's staked or swaged in, and over time, especially if the pivot has stiffened from rust, it will loosen, and eventually will pivot there instead of in the outer frame. When this happens, the window will flop and not stay open. It's pretty common in rust country. There's not much you can do to fix it, since you can't weld with the glass in.