Tokyo is on my list to visit, hopefully during the blossom season next year!
If you visit the snoopy museum, you can pick yourself up a Bicycle Snoopy deck of cards. Although the museum isn't located in Tokyo/Osaka. You can get the cards online, but if you plan to visit, you'll save on some bucks.
Try Ginza Itoya, they tend to have playing cards of sort. They initially launched their own Bicycle branded deck, you might get lucky and locate some original ones, as they're fairly sought after. They released a second one too.
Rilakumma store will have some, as like the ones above, they released their own Bicycle branded deck of cards in the past.
Best of luck and happy & safe travels!!