I read that Spanish and Portuguese missionaries held Mass in Japan in the 16th century. Where did they get the grape wine and bread necessary for celebrating the Eucharist when there is none of it in 16th century Japan?

Japan traded extensively with foreigners which would have brought bread and wine as well as their manufacturing techniques with them to Japan.

The elite of Japanese society in 15th and 16th century Japan had a decent amount of access to quite a lot of wines brought from overseas.

Bread would have been harder to bring over directly but Japan had grain flour in great supply and could have easily made unleavened bread for Eucharist.

It should be noted that Japan had a very strong grape growing culture, but specifically for eating the fruit, not for wine. It was a great luxury to have great, juicy grapes at your table for the Japanese elite of the time.

It would not be until the Meiji Period several centuries later where Japanese vineyards would even try to make wine. There simply wasn't any interest in it during the 15th to 18th century. And of course, the ban on Christianity and grape wine's association with the religion could not have helped!