I've done lots of move out cleaning over the years. Some units are pretty clean, others are absolutely disgusting, just depends on the people who lived there.
Property managers can provide both volume and recurring work, and they are always looking for new cleaners/cleaning companies to work with. I worked for one as an employee for a few months, lots of work (more than full-time if I wanted it) but the deadlines and the pay didn't make it worthwhile. I've also worked for a couple of managers as a contractor. Typically they like to pay a flat fee for every unit rather than have you quote each job. As you can imagine sometimes that means you don't make much per hour and other times you make a lot.
I've also got a lot of work from a simple ad on free classified sites. Those ads pull calls from property managers, owners, realtors, and tenants so it's a mixed bag. If you want to land most of these jobs you will need to reply FAST and have a crew ready to go at a moment's notice, I find most people call the day they want the property cleaned.
A related line of cleaning that I've done quite a bit of as well is construction clean up. The best is if you can get in with the general contractor, it's recurring work, although not very frequent unless they're a huge company. And if you're reliable and do a great job they pay VERY well. I particularly like working for contractors who do major renovations on houses and offices, you can often pick up the house/office cleaning from the owners as well.