The PHB (p.125) states that you can either take the equipment package provided by your background or you can use coin to purchase your starting equipment. If you choose to purchase your starting equipment, then you can not also take the equipment package suggested for your class.

So my question is this: If I choose to buy my starting equipment, is there a way to obtain/purchase items such as the urchin's small knife, pet mouse, and map of one's home town? (And is "a set of dark common clothes including a hood" considered the same as regular common clothes for the sake of buying equipment?)