Tokyo is a mix of all different kinds. When it comes to shopping, there are department stores, supermarkets, big electronics stores, and little shops all intermingling together in some kind of strange cosmic balance. There are even a few street markets to be found in the major cities.
Ameyoko is found just south of Ueno Station, on either side of and under the tracks leading to nearby Okachimachi Station. It's a busy place any day of the week, certainly helped by the large number of tourists who want a glimpse at Japanese goods.
I'm not sure why I don't have any pictures of the clothing stores, of which there are many. There aren't many actual souvenir stores (check underneath the tracks?). The west-side road closest to the tracks seems to have the most interesting browsing available, though the other west-side street and the east-side street can be explored as well. There is at least one 100-yen shop, drug stores, suit stores, and plenty of western wear stores with way-too-high-priced goods imported from America and possibly made in China.
Ameyoko is easy to find by following the tracks south from Ueno Station. As soon as you cross the main street just outside Ueno Station (while you're under the tracks) you're there. Go to the right for the west side. Just remember: those western clothes you see in the stores can be had for much less back home, so just focus on the Japanese stuff. Engrish t-shirts aren't tough to find.