Hi. The best places to find stamp rallies are at train stations, as many are run by train operators. Look at the posters and flyer racks that you should find in almost any station. You can also check flyers at tourist destinations and tourist information centers as they might include a rally. I've also found some in konbini (7-Eleven to be more specific). Just keep an eye open - you might even find a stamp and discover the rally at the same time. Also, it definitely is quite hard to look for permanent stamp rally held each month, so you're best bet is to monitor all possible stamp rally that is about to be held.
Another alternative to stamps rallies is to collect temple and shrine stamps, called goshuin. They also have pilgrimages that will bring you to see to a hundred temples and shrines depending on the pilgrimage of your choice. Just remember, do not mix commemorative stamps and goshuin.