Even local musician classifieds are not very fruitful...unless you live in the major cities, and then you end up with strange people contacting you.
I've tried it....found one I was working with for awhile last summer, and then she flaked out, like most of them do.
I haven't gone this route yet, but I think a better option is to go to some band gigs where there are female singers, or attend some open-mic gigs where you get lots of people....then when you spot/hear a female singer that fits your needs....aproach her with 1:1 offer. Tell her you need some vocals, and in turn you would be willing to help her out on any tracks she might be recording....etc.
Or...just grab your balls and sing higher....