There are several websites which you can visit to get an overview of organ concerts in Paris:
Calender of concerts in the region of Paris
Calender of Paris les orgues
Calender of the catholic church of Paris
When you visit Paris once only, attending the masses and a few weekly auditions (small organ concerts) is the only way to hear the most important organs. A tough schedule [482 KB] for your visit on saturday/sunday is necessary!