I am sorry that it has taken me so long to get back to you. Your best bet, for such a large quantity is to find a wholesaler who will deliver a truckload to your house. You will need 3 cubic yards if your planters are 2 feet deep, or 4.5 if you go 3 feet deep. This is based on your assumption of a 50 square foot planting area. A cubic yard equals 27 cubic feet, if that helps. A standard pickup bed can hold about 1.5 cubic yards easily, if you have a truck and want to save money on delivery.

You are looking for a locally owned landscape supply store. They will usually have giant piles of mulch and rocks and whatnot that you can see from the road. These stores will sell a "garden mix" of some sort that will be absolutely perfect for what you are trying to do. This will by far be the lowest cost and simplest option. Being in a dry environment, you should be using lots of mulch to help conserve moisture. Make sure you plan that into your design.

That being said. If you are looking on advice on rock planters in a survival SHTF type scenario, I would say forget planters, especially in a hot dry environment. They get hotter and drier than the regular ground. You want to be planting in "ditches" and low spots, if that makes sense.