I must be tired. I just wrote about a two hundred words on auditions and then accidentally deleted it.
Here are the edited highlights
1) If you've got a budget cast via agents. You'll end up seeing people who meet your casting profiles and who are good enough actors to have acquired an agent. If you haven't got a budget, some agents will still help you, but only if you've got a good reputation in the local industry and they want a less experienced actor's name linked with yours.
2) Every major city has an Actor's Centre, a place that professional actors use as a resource centre. You'll find a willing and more than often free cast here, plus they'll have rooms you can use for the audition.
3) Drama schools - some have a no professional work whilst training policy, most don't. You'll find lots of young, inexperienced actors. A mixed blessing as they are also often unreliable.
Good luck