Not everyone photographs people, so your model may not actually be a person or living being. To be very broad with the term, the "model" is the main subject of your image. If you shoot landscapes, your model may be the location or specific terrain you're photographing. For architectural photography, your model could be the building or room that you're shooting. I photograph a lot of food, and I like to consider the main food item as my model, as shown in Figure 3.
Figure 3 I often consider my main food item as the "model" in my food images. This piece of salmon was carefully selected and prepared so that it looked its best for the photo.
The point I'm trying to make is that when planning a photo shoot, even if you know what your subject will be, you still need it in front of you in order to photograph it. I might want to set up a lifestyle shoot with two or three models and have the entire thing already planned out in my head, but unless I know exactly who I'm photographing, I can't make it happen.
Finding models can involve searching online, networking, shopping, or just scouting at a location. What you end up using as your model depends on the concept of your photo shoot and the overall look you're trying to achieve.