You'll have to check around for compatable parts, but you could actually make one. All the big box stores (depot, lowes, etc..) carry the corners and rails, and a small variety of the mechanical parts, such as spring catches and the tab that slides in the track. The parts fit together pretty easily.
Of course, the first thing to is see if you can find a label anywhere on the windows. It's possible the manufacturer may still be around and able to make one and UPS it to you.
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