This is a common question amoung stock photographers. We are all told that images with models are better than images without. Being relatively shy about asking people to pose for us we often start with family and friends.
Eventually they get tired of our ever present camera and it forces us to re-examine the effort. We then ask friends to find us more friends. Sometimes this works and sometimes it doesn't. When it doesn't we feel stuck.
So now what do we do? There are few options available.
The first option is find a local agency. The upside to this is that the models are usually reliable. They don't want to loose their place in the agency so they are careful to follow directions. The downside is that not every agency is really an agency.
Many "agents" are nothing more than a social secretary for the girls in their group. If this is the case you may be paying a lot of money for a model who is not really up to the standards you were hoping for.
The other issue, of course, is the money. Agencies usually charge $50 to $300 an hour for their models with a 2 or 3 hour minimum. This is not true in all cases. Some agencies will send models out on "test shoots" where the model is basically auditioning for a later shoot. These types of shoots are done for little or no money paid to the model/agency.
You will want to check with your state about licensing of agencies to ensure that you are dealing with a legitimate business if you decide to go this route.
Most microstock photographers just don't have the funding to pay big bucks for models or operate in an area that is to remote to have an agency within a practical driving distance.
For these situations you can join model/photographer social sites like Model Mayhem or One Model Place. Model Mayhem is free to join but OMP does have an annual fee.
These places make an effort to screen members so that only people who truly have a desire to be there become members. You can browse their portfolio to see if you like their look and many models will work TFCD.
This means Time For CD. In other words they will model for you as long as you provide them with a disk of the finished images. The catch here is that you will still want to be clear on how the model can use the images. Most will simply want to post the pictures in their online portfolio but others will want to sell posters or greeting cards using the images. Just make sure that everyone is on the same page before the shutter starts to click.
You can also post casting calls on these sites. It's best to check any references or other information that they post in their profiles. Because there is no commitment to the site on the part of the models or the photographers you can sometimes get a model who doesn't show up or who arrives completely not ready for the shoot.
In that same vein you could post a casting call on your FB page, My Space page or Craigslist. Again just be explicit is detailing what you want and what you are willing to give the model in exchange. And always spell out up front that you will require a model release. It's amazing how many "models" are not willing to sign a release.