This is a very typical legume in Puerto Rico that is used primarily in making "Arroz con Gandules" or Rice with Pigeon peas. You can either find it dry (which would require soaking overnight) or get it from a can. You should be able to find this in most grocery stores or supermarkets under the Hispanic section. Otherwise you can also get it from Amazon.
Below is an image of the difference between dry gandules (top spoon) and soaked gandules (bottom spoon). The process of turning dry gandules to soaked gandules is pretty simple. Pour the dry gandules into a bowl with some 2-3 table spoons of salt with 1-2 quarts of hot water. Let is stand overnight. To make sure they are done, you should be able to pick any gandule and squeeze it between your fingers. For simplicity, we recommend to use canned gandules if dry ones are not easily available or you don't want to spend time soaking.