turbinado is just raw, unprocessed, or demera sugar. I think the most common brand found in the supermarket is "sugar in the raw" or something like that. I've never not found it in a grocery store.
However, when I don't have it on hand, I just do a one-to-one substitution of brown sugar. There is probably a more precise quantity substitution, but I'm not to precise on rubs in the first place...