My only concern would be that commercially available vocals (Loopmasters et al.) are going to be rinsed by the amateur and/or "paint by numbers" crowd.
There are so many other options--options I feel are often ignored because they take some effort or minor financing--but IMO...
1. If you live near-ish a college or university put up fliers on campus calling for a vocalist looking to work with a composer/song writer.
2. There are are several online networking sites where vocalists create a profile and pricing scheme to work with musicians/producers whatever. One is http://vocalizr.com/
It can't hurt to give it a shot.
3. Depending on the extent of the vocal you could try recording something yourself. I know it isn't for everyone, but if all you need is a phrase/hook...it could be an answer.
4. As someone who co-runs a small label...I wouldn't be adverse to someone sending a demo with "place holder" vocals.