I've ordered from Hobby Link Japan at
http://www.hlj.com before. Some people on the board have mentioned problems with them, but my order went just fine. You have to realize before you order though, that it can take a while to get your order. It took mine about 3 weeks to arrive. I got one of the yellow bB's (57mhz) from them for $10. Can't beat that
Right now they have the black bB on sale for about $12
http://www.hlj.com/cgi-local/hljpage.cgi?TOM62852 Unfortunately it's 27mhz so it won't give you an extra frequency in addition to your amercian cars. They do have a bunch of other cars for $23-$30.