Replacement armrests for a Chevrolet Silverado are available at a local Chevrolet dealership as well as online at GMpartsshop.com, Newgmparts.com and Autopartswarehouse.com. The older the model of the Silverado, the less likely a dealership is to have new replacement parts.
Finding original equipment parts, referred to as OEMs, is best done at the dealership. However, there are many aftermarket parts resellers that can provide OEM or OEM-equivalent Chevrolet parts for prices that are reasonable compared to the dealership. Websites such as GMpartsshop.com and newgmparts.com are resellers along those lines. Another option for locating a replacement armrest for a Chevrolet Silverado is a local junkyard for salvaged parts.