We’ve been able to find many things in Tokyo that an American expat would want. There are a few things, though, that you would want to prepare yourself for before you come. 

There are no Taco Bells in Tokyo, except at the American military bases.

Shoes are very difficult to find in larger American sizes. The styles and widths of shoes are also hard to fit for Westerners in general. If you are coming to Japan for any length of time I highly recommend buying several pairs of shoes before you come.

Larger or medium size Western clothes are very hard to find. For some items (such as underwear), Don Quixote does stock some larger Western items if you can find them.

American toiletries are very hard to find. However, you can usually find something that works just as well in an American brand. it helps to have a Japanese friend, or if you speak some Japanese. If you have a particular product that you like, you probably want to bring it with you. The Nissin grocery store near Azabu-Juban and the National Azabu store in Hiroo do carry some Western goods, although they can be very expensive ($25US for a modest size box of Cheer Free for example).

You cannot buy Sudafed in Japan.

M&Ms are hard to find in Tokyo but they do exist. Same for Mountain Dew. I have never seen 7up here in Tokyo but I can find Dr. Pepper. There are many other types of soft drinks available here, plus many drinks that would be hard to find in the US. Canned coffee is particularly popular here, and the vending machines sell both hot and cold products, including corn soup which is fairly easy to find in vending machines. And yes, there are beer vending machines in Japan. 

American breakfast cereal is not at all common in Japan. Of all the possible choices, I see Frosted Flakes the most often. However, there are European breakfast cereals in many grocery stores, though again not a lot of choice. 

For many of the above items, The Foreign Buyer’s Club in Japan can supply most American grocery store products, although the prices are Tokyo prices and it may take a few days or weeks.