The Atari mouse was quite poor, and very few remain in good working condition after the years. When it finally needs replacing, these mice are very hard to find. Try contacting an Atari dealer in your area, as most of them carry some sort of Atari compatible mouse.
???????A PC serial mouse can be connected to the serial (modem) port???????. There are two drawbacks to this. Firstly, on a machine with only one serial port it prevents from connecting anything else (modem, extra midi ports...). Secondly, you will need to load a serial mouse driver at each boot. Mouse25 is such a driver and can be found on your favourite FTP site (see section 2.2.3).
If all else fails, you can always use the Alt + Arrow keys trick (Alt + Shift + Arrow keys for pixel scale movement, Alt+Clr/Home for left click, and Alt+Insert for right click) as a lifesaver, but that's hardly a practical solution in the long run.
Some Amiga mice have a little Amiga/ST switch allowing them to be used on an Atari. There is reportedly a means of converting an amiga mouse to an ST mouse by inverting two wires, but I could not not find the instructions.
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