It's not the only way.
Lots of collaborators can deliver broadcast quality vocals too.
I would hire a demo singer in those instances where I already have written the lyrics, melody and recorded the music, because at that point it wouldn't really be fair to involve a collaborator (for either parties), allthough you can of couse still choose to do so, which some cases may justify. But at that point you are pretty much married to what you have, and need to follow that trail to the end (which may well be the first submission that states that the lyrics need a serious rewrite

). At that point you start to wish you picked a collaborator that would have debated the issues with you at a much earlier point. Often "professionalism" is an enemy, if it means that they will just take your money and do what you want, without addressing important aspects of your song. As songwriters, our mouths are too often much bigger than our ears, and it does us no good.
So, in most scenarios I would much prefer to collaborate. When you can't sing the song yourself, chances are that the input from a singer would improve what you have, in terms of phrasing, words and even melody. But also when you feel too secure on your stuff, it's great to have more opinions to compete about desicions, imo. And it's by far the most flexible, when you need to rewrite. Great songs write themselves, but most of them are just good, and a good song is a rewritten song.
What is considered a professional vocal also depends on the imagined end use. For film music a mature 50+ woman with an untrained voice that shivers can be exactly what works, for a tv track a nasal midaged man with poor english can sell the ticket, and for a Nashville demo, a voice they haven't heard already on a hundred demos, can be refreshing for some publishers.
Of course, demosingers can work well in some scenarios too, so you have to be real about your goals and what you have. Here's three of the "official" places to find demosingers. Of couse many in these forums would do demowork, I'm sure, but also surf Soundcloud, Bandcamp and Reverbnation for singers you think have a sound that will suit your production and the end use you have in mind. Many demostudios also have a list of available singers on their website, so you can just send in your track, and they will record to that so you just have to pay for the singer.
Purevocals.com
http://www.purevocals.org/
Demosinger.com
http://www.demosinger.com/
Singerspro.com
http://www.singerspro.com/uk/
Remember to talk about terms, and send them a work-for-hire letter/contract, in advance.
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