Make a list of all foam props needed for the production. By identifying your needs up front, you can get the most use out of the material you order, with less waste on a tight budget. You will also be able to manage your time better, since you won't have to shut down prop production because you ran out of foam.
Write down the dimensions of each prop. Generally, foam is either molded on site or ordered in specific sizes and shapes and then sculpted by your prop technicians. Order foam that has greater dimensions for height, width and depth than your prop.
Find a foam dealer. These companies will ship you foam blocks in the size and shapes you specify. Some of them will create custom-shaped foam blocks for you to paint. Others will create the props for you at their factories and ship the completed props to you. Analyze your available manpower, skill set, equipment, budget and time available to decide how much of the propmaking to do using your own production staff and how much to outsource to propmaking professionals.
Order the foam needed. Some types of foam do not lend themselves to further carving after the foam has left the mold. In some cases, you will need to have the foam shaped to the prop.