Where can you buy a usb cable?

if you buy a dsi action replay it comes with an usb cable it fits in the action replay

You should not have to buy a cable and the cable the dsi comes with fits the dsi perfectly. it attaches behind where the game pak will go.

The question is specifically for the DSi, not the Action Replay. See the related answers.com question "where do you get a usb cable for action replay?"