Extra trays are available for several models of Nesco American Harvest dehydrators, as of 2015. The trays are available in packages of two and are sold by Nesco, Amazon and Bed Bath & Beyond.
American Harvest dehydrators expand to hold seven to 30 trays, depending on the model. The dehydrators automatically adjust the amount of drying pressure to accommodate extra trays, and it isn't necessary to rotate the trays during the drying process. Nesco also offers a product called Clean-a-Screen, which is a reusable mesh insert that keeps small items like herbs and diced vegetables from falling through the trays.