I'm having a hard time finding superfine sugar. It's not something buy often, but I seemed to remember seeing the boxes of Dominos brand superfine on the shelves. I'm in DC and have many choices of stores, but I didn't think Williams-Sonoma or Sur la Table were necessary. Has anyone else noticed this?
Make your own. Grind regular granulated sugar in a coffee grinder or food processor and voila, superfine sugar.
Edit: ipsedixit beat me to it.
I buy superfine sugar at Safeway. Wholefoods have them too. The brand I see most often is C&H. C&H also labels it as "baker's sugar. You can also throw some granulated sugar into a food processor(if you have one) and give it a whirl. It's also labeled as castor sugar.
And sometimes labeled "bar sugar" as it dissolves better than granulated in cocktails. I doubt whether you'd find it labeled castor sugar in the US though, that's a British designation, but it's good you mentioned the term, as it can be confusing when baking from a UK recipe that calls for castor sugar.
I find superfine in most supermarkets right next to the brown/confectioners sugar in the baking aisle, and haven't noticed it missing from the shelves lately.
To the OP - ipse's make your own is really the best thing to do, certainly if you can't find it in
the markets. Granulated sugar is usually cheaper than superfine, and it's the same sugar, just ground more finely.