E bay is a good source. If the trailer has a single skin, line the interior with pressure treated plywood to prevent outward dents. Make sure the ramp is robust enough to hold the animal's weight. Depending on its interior surface, you might consider the installation of a slip proof ramp surfacing. Install forward and roof vents, with drop down windows. Adequate ventillation is paramount.

Check the entire interior for sharp edges and protrusions that may injure an animal. Install an interior light(s) and tie rings. Determine the age and condition of the tires, determine the last time the wheel bearing were packed and renew if necessary.