I am wondering if what you called "C-potassium bicarb" is not in fact Buffered Vitamin C with potassium bicarb. If yes you find other brand of Buffered vit-C. And some are buffered with Calcium, Potassium, Sodium ... and maybe preferable to just buffered with potassium. It is important to have the proper potassium/sodium ratios.
But whatever kind of buffered vit-c in powder there are, they are expensive, but think that if you buy them in capsules, you would probably pay twice or three times.