Cereals are widely available at every store in Spain, unless you specifically need to eat some weird kind of cereals. The most popular ones are available
Kellogs .
As for the rest of the things, Puna is right.
El Corte Inglés has a wide range of international products. You may find there what you are looking for.
About the prices,
El Corte Inglés has a selection of products on offer every week or every two weeks. In such cases they have very good prices.
Unlike what Puna said, I've always found their products to be fresh and with high quality.