In Japan they sell hand-held radiation detectors in electronics stores like Bic and Yodobashi (think Best Buy on steroids). They're in the little section with the first aid kits and earthquake alarms.

I know Yodobashi ships some items internationally, so it might be worth checking both of their web sites. They're just hanging on the wall in blister packs, so they appear to be cheap commodity items. Maybe $50 is my guess. I never looked at the tags.