Welcome to the forum Poppi!
I only use powered sugar for top bar dusting (varroa count/control), I personally do not do the sugar shake unless there is a lot of trash on the sticky board and I do not trust my count. I have found the powered sugar with out corn starch, in regular stores but it is not the generic (so you get to pay more). As far as fondant, I use regular sugar (comes in 25 lb bags) because it does not cost as much. -Mike