I would suggest you do your homework, read through the trade magazines. You might find ads soliciting modelers. If you're going to do this as a business, then you'd need to do up a promo sheet of your work and send it to the firms you want to do business with. You might want to even have a website.
The few models I did, I learned about the projects from the reading the newspaper and contacted the firm directly. The first couple I had to show them examples of my work, so I build a model of my garage and the proposed remodeling I intended to do. Simple yet effective. Most of the models I did I built in 1:160 so the building were small enough to carry with ease yet detailed enough to make the point.
Some firms use their own in house resources (interns) using foam core board to make simple structures with printed details attached to them. Effective but not as good as using styrene which allows for more texture.