No doubt it is very tough. Just look at the census, only 20 graded copies, mostly low grade. Likely the hardest to find DC is something a bit more esoteric.
Look at Three Mouseketeers #1 also by Sheldon Mayer. Only three copies on the census. I think Sugar and Spike seems rare because it is considered a key and is collected by enough people to notice its scarcity. , where as many, many DC's from the time period don't have the same following:
Any ways, S&S early issues are HTF, and whether they are the rarest or not doesn't really matter to those of us who love them.