Without pics, I would say use the same material. But usually pine or oak if stained, maybe poplar or pine for painted. You will probably have to remove trim under the sill as well. Pry carefully so as not to damage walls or trim.
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Is this an interior flat window stool with apron under it?
If so, take off apron to see how it is attached. You should be
able to pry up with a flat bar. Cut caulk around edges with a razor knife
to help prevent wall damage and make it easier to pry up.
Your local lumber yard should stock or be able to order matching material for you.