Old Thread but found a Retailer for Spectraquartz that sell 50# bags for $19.00. http://www.questbuilding.com/
Considering its $16 at McMasterCarr or $20 for 75 there is not much price difference between the two. All the local places in SoCal that do sand blasting use copper slag and not coal slag.
I am using the black F-grade and really like it so far.