stacy, can you link the recipe? your link gave me a list of 19 recipes.
I'd go with granulated, turbinado or demarara sugar. The Chinese rock sugar comes either white or yellow, depending on how refined. I'm not certain on quantity to substitute, but I'd start with 7 oz of sugar, then taste for sweetness (the typical sub is 1 T sugar for 1 lump rock sugar, and the rock sugar is reportedly less sweet than regular sugar).
stacy, this sugar is only being melted into the sauce and I just don't think that rock candy has a very unique flavor profile that makes it a requirement. My guess is that Ming wanted to honor his parents' use of the sugar they grew up with and used in their recipe. So I agree with hardlikearmour's advice, but, given the expense of other sugars, i'd suggest you just use regular old granulated white sugar. Neat recipe; i'll have to make that;thx!