What is a good substitute for spanish sherry?

Depends upon why you are substituting. Is it because you don't have any Sherry on hand? Or is it because you want to avoid the alcohol in your dish? If it's the second reason, Dr. Pepper (especially if it's gone flat) replicates the flavor nicely. If it's the former, then it would be helpful to know what the dish is in order to suggest the best substitute.

lily, you first need to tell us what you are making and some of the other ingredients.

You can substitute Madeira or Marsala in a pinch, or even apple cider vinegar if you're lookin for a non alcoholic version