3M doesn't make Colorquartz any more, but it appears that another company started making it in an almost identical fashion, including the same colors, same two grades (named differently though: F-grade is the same as Colorquartz's old S-grade and Q-grade is the same as their old T-grade), and same distributor. It's called Spectraquartz now. Here's a link to their website:
http://www.spectraquartz.com/
You can get it from a number of places, one of which is N.T. Ruddock. Their price list is found
here. Notice that it's about twice as expensive as the old Colorquartz and you can get quotes for blended mixes of different colors just like you could with Colorquartz.
It's also worth noting that, although they have a picture of them using it in an aquarium (I think they re-used the old Colorquartz pic), it does not say that it's safe to do so like the old Colorquartz did. I know they are applying the color to the quartz in a supposedly different manner, although their description of the process sounds the same, just with different wording. It is still guaranteed to be chemically inert and pretty much permanent (so it shouldn't leach into your water like some Walmart substrates

).
IF this is pretty much identical to Colorquartz, as it appears to be, I highly recommend it - but I have not tried this new stuff, so I can't say that it is the same. I love my Colorquartz. It's heavy so that it doesn't get stirred up into the water column often, and when it does, it settles quickly. It doesn't fade. It doesn't change your pH. It's non-magnetic. It's not sharp (at least not the smaller grade). Basically, it's substrate heaven in my mind.
Edit: ^ he beat me to it. Footnote: the price list on N.T. Ruddock is for 50# bags.