See if the Jacks Magic product is available:
Jacks Magic The Pink Stuff 1qt (60% HEDP) @ Amazon
20% KemTek HEDP @ Amazon
Note that this particular Jacks product contains 3x the amount of HEDP per quart that the Kem-Tek does.


Also, somebody found a giant size vitamin C at Amazon:
Ascorbic Acid Vitamin C Powder USP Pharma Grade 11 Lbs
That's probably more than you need, but it should keep if you store it carefully.