They don't make it anymore. You may be able to find some in stock at a place that carried it, but it's been over a year since they stopped producing it. I think Estes makes a similar product.
You can always try Spectraquartz. It looks identical to me. My local distributor in Cleveland may still have some Colorquartz and as they run out, they use Spectraquartz. It comes in similar colors. I've got both in my tanks.