There is an excellent one, made by Magnum Dynalab, called the Signal Sleuth. As it happens , there is one offered here at A. Look under http://cgi.audiogon.com/cgi-bin/cl.pl?misctunr&1028827506&class&3&4&.
No, I don't know the seller, but I use one with excellent effect. Cheers,
Detlof