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To my knowledge no aftermarket replacements. If you want OEM, you will need to call a GM dealer. Otherwise, a clip from a junkyard or depending on the repair, you might be able to use a front fender.
If you're trying to repair a minor rust spot you can use a front fender from the opposite side for metal. Although the shape isn't exactly the same it's pretty close. Just cut what you need from the fender for patch material.