I got two sets of used 2012-2014 Toyota Yaris knobs on eBay for ~$60.00 shipped each (for the three knobs and the Yaris HVAC bezel). Ours weren't broken or anything, I just wanted them for their chrome trim after seeing gtv's post (linked in zbeyers' comment above) about them. They're not too difficult to replace.
It does appear this particular individual knob is still available from GM
You could also check out car-part.com
. You may be able to find one in a junkyard which would obviously be cheaper.