here's what i can find regarding pre-paid sims for Japan. there's a few pre-paid sim carriers, but from what i can tell, the others require you to use their phone or device. with the exception of this one
http://mb.softbank.jp/en/prepaid_ser...paid_plan.html
and based on the conversion of $1 - Y87.44....a pre-paid that cost Y3,000 costs about US$35.