[Nyororin]

I am sure there are cheap ones on sale in various places around the net, but I have posted in the past about the pointlessness of buying a detector, particularly a cheap one, in the past.
A poorly made, poorly calibrated detector is worse than worthless... And a proper one is going to cost you quite a lot, even outside of Japan.

As for finding one in Japan, if you are in Osaka - have you tried looking around Nipponbashi?
However, if you are in the Kansai region, I don't really think having one is going to do much for you....