We have built up a collection in a variety of ways:
We have received a few books as hand-me-downs from friends of friends. That is actually how we received our first book, a little peek-a-boo book about a bear. We also recently received two books from a preschool classmate of a friend.
We drove to Seattle and went to Kinokuniya which, while far, we combined with a trip to the children's museum. We've gone twice now. I like going in person because I can more easily choose books I know B will like and that I know I can read.
He loves this series. I'm still hunting for this one but we've got 4 others and I've managed to find other books by the same author and about the same character on Amazon.ca.
We've bought a few books from World Kids Books . The website only lists French, Spanish, Russian, and English books, but there were other languages as well. They carry very few Japanese books, but we found good ones.
I've ordered a few books from Amazon.ca. I haven't been as successful with this, there just aren't very many listed. (Well, listed for reasonable prices).
A friend brought back a bunch of good books for us when she went to Japan. Books are heavy though, so it's difficult to bring a lot.
We ordered a nice series through B's Japanese school. We haven't received them all yet, but this seems to have worked out fine.
We found a few books in our recycling room. Someone was moving out and they left a big box of books. My husband found them and brought them up. We lucked out as there were several books from a popular mouse series.
Once a year, there is a used Book sale at the main branch of the library. One year I found a couple of children's picture books in Japanese. Most years I haven't found much but there is always the possibility. (Plus I found some good books in English anyway). There's also a used book sale at the Japanese cultural centre and my sister has had good luck finding children's books there. We went together, she just found better books.
And last week, B's friend gave him a Japanese picture book as a gift! It was very thoughtful because the friend chose a book that he himself enjoyed and asked his dad to bring it back from Japan.
So we have ended up with quite a few books.
They get a bit messy because B likes to look at them
We also can borrow books from the library, but only the main branch carries languages other than English, French, or Chinese. We went more often last year when B was still in preschool so he had days free. I've seen other parents borrow books from B's Japanese class, but I want the books at school to be fresh so that he will sit quietly during the time they have to look at books after snack. (As they finish eating, they are excused and can sit and look at books.)
We have been pretty lucky, both in being able to afford the books we've bought and in receiving the books we've been given.