Just because your Swiffer or spray mop is leaking doesn't necessarily mean you need to get a new mop. A good percentage of the time you just need a new refill. Here is an easy way to check to see if you need a new mop or just a new refill.
Remove your bottle from the mop and then hold it upside down.
If you see it dripping while it is upside down (cap side toward the floor) then their is a good chance that the seal on your bottle is what is damaged and you just need to replace your refill bottle.
Also note: If your spray mop bottle is empty you may be able to refill it with an alternative floor cleaning solution. See: How to open a swiffer wet jet bottle.