I have to concur with the folks above on the Noordermarkt on a Saturday. Great organic and artisinal produce, meat, cheese, bread, etc There is also a great little crepe stand in the midst of the stalls!
If you are looking for for a grocery store, Marqt on the Overtoom is close to the tourist areas and has good selection and great quality food. (Prices to match in many cases as well) http://www.marqt.com
For fashion, hit the 9 streets for a break from the mega brands. The "negen straatjes" as they are called in Dutch house lots of local designers and boutique fashion stores. http://www.de9straatjes.nl/