If the mouse is still in warranty, and the charger has stopped working, they have to honour the contract which you get when you purchased the mouse, which is either fix it or replace it.
However, that route will only work if you purchased the mouse have the receipt and its within warranty.. and the charger has broken not just been lost.
The charger you are after is:
Logitech AC Adapter
Model: 190162-0000
Input: 100-127V ~0.3A
50 - 60 Hz
Output: 5.8V === 1A 5.8VA
uses the same charger as the mx700 from what I can tell.