As well, when I was in Japan this time last year, buying a 3G SIM Card in Japan was not possible for non-residents.
The only option seems to be renting a card at the airports, which was not a cheap option.
If you buy any SIM card in China, you will have access restrictions.
If you buy a prepaid card in China or Japan, you will not be able to roam data in another country for any reasonable amount of nmoney. The reason is that the vast majority of the customers want to buy a card only for that market.
Ironically, you will have more luck abroad to buy such a card. Regarding the international roaming for example, you can buy one in the UK for 8 pounds a day. Regarding the firewall, you will not get any guarantee that it will work. I use a contract from Hutchison in HKG and can somehow access all normally blocked websites when roaming in China, but no provider will advertise this at the risk of getting blocked.
So if you cannot find an offer abroad, you will have to buy one in each country, and chances are that you will not be able to circumvent the firewall. Unless you get a VPN service and connect through it.
Credit:stackoverflow.com‘