Importing from Play-asia doesn't cost a lot of money. It only costs about five dollars in shipping. But it takes a long time to get it, about a month.
Also, the Japanese games are region locked, so if you only have a DSi that you bought somewhere other than Japan, you won't be able to play it.
Also, the Japanese games are region locked, so if you only have a DSi that you bought somewhere other than Japan, you won't be able to play it.