The original arm guards were made mainly of animal hides and even stone when bows and arrows were first used. Fortunately, a wide variety of materials are effectively in use today. The materials that are most common are:
Leather (suede)
Cowhide
Materials from composites
The most prevalent is going to be the composite material guard. These are found all over the internet and in archery equipment-carrying retail stores. All three, however, provide the arm with great defense. It's going to come down to taste, what feels more convenient, and what the best price is.