I've found one from an online GM dealer online for ~550 including shipping, gmcertifiedparts.com (but I've found multiple places with the price within $20 or so from each other).
Before mounting it, I took a soldering iron, and melted a solid seam along the whole perimeter of the mounting surface, and also poured epoxy around the little vertical stands that connect the upper and lower parts of the spoiler. I hope this one will last better.
The plastic of the spoiler is actually very resilient. I've tried to break a piece off the old spoiler, and I couldn't, I could only cut it with the shears, and even that quite difficult. But I guess the loads on these little connectors are so high that over time they break off.