Convenience store solves all
If you find that at some point you have no more underwear in your luggage, or you forgot underwear altogether, never fear for there are easy solutions.
My favorite convenience store, "Lawson 100" is a slightly different convenience store that can be likened more to an "all around" food store. With a wider and less expensive selection than a regular Lawson, or Family Mart, or 7-11. Here, you can find bike equipment, electronics, toys, food and drinks, but there is also an array of inexpensive clothing for emergencies.
White shirts, socks, and underwear for both women and men will only run you 100 yen! This is a perfect solution if you find that you ran out of clothes before you had a chance to do any laundry!