Located between Mezcales and Bucerias the MEGA grocery store offers everything you can imagine. They have a fresh fish section, meat counter, bakery, groceries, clothes, electronics, etc. Try the local brands rather than the imports, you might be surprised by the quality and you will save because of their aggressive pricing!
TIP: The people that bag your groceries make their money only income from tips, so a few pesos would be appreciated. For a $500 pesos bill, $5 pesos is fine.