What is the best place to buy food online?

That depends on what's available where you live. Some supermarkets allow you to buy your groceries online and have them delivered.

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It really depends on where you live. Here in Thailand you can order food online and pick it up the same day at Big C Supermarket and Tesco Lotus Supermarket. Also if you are in the Bangkok area there is a great Italian Deli called DOLCE VITA that you can shop online and they will deliver. But don't expect it to be cheap.

Probably right in your kitchen .... look for items that you are low on and order them at the same time .