Possible to buy passes on arrival in Japan?

I am arriving in Japan on April 16. My itinerary is not yet firmed up but planning a month of travel with the first 5 days in Tokyo. Is it possible to buy the whole country pass or regional passes once in the country with the tourist visa stamp in my passport, or does this have to be arranged ahead of time? Since my itinerary is flexible I don’t know yet which passes I’ll need.