Employees at rental car companies are trained to say that they cannot guarantee a specific model for your reservation, except for Hertz, which has special "collection" models that you can reserve. Finding, say, a Ford Mustang or a Chevy Malibu will likely never present a problem. To locate and reserve something less common, though, make sure to call the local office (not the national reservation number) and ask if the car you're interested in is available on their lot that day. If it is, you may want to jump on it right away. And if you can't get satisfaction from the first person who picks up the phone, ask to speak to the manager, who will consider it a customer satisfaction issue. If you use a particular company with some frequency, mention that.
Once you get your vehicle of choice, determine whether its trim level (the designation of a model that includes specific equipment) is the same as the exact vehicle you're considering buying. A difference in the trim level might be minor, such as chrome vs. aluminum wheels or leather vs. cloth seats, but it could also mean the difference between a four-cylinder and six-cylinder engine. Don't make color or specific features an issue; the goal is to see how the car functions and drives in a variety of day-to-day situations.