Finding appropriate substitutions for missing ingredients is one of the great talents in a baker's skill set. Long experience is the best guide, but earning that experience involves a degree of trial and error. Usually, looking for the most similar ingredient available is a good tactic. For example, if your recipe calls for rice bran and that's not readily available at your local stores, you can substitute other types of bran.
Grains all have a broadly similar structure, with an oily "germ" at the middle and a fibrous husk of bran around the outside. The remaining portion, accounting for most of each individual grain, is called the endosperm. Pot barley and white rice are familiar examples of grains with the bran milled away, and white flour has both the bran and the germ removed. Bran is often used as a baking ingredient in its own right, adding B vitamins and fiber. Rice bran is an especially good source of fiber, and also adds a pleasantly nutty flavor to recipes.
Oat bran is arguably the best substitute for rice bran, with the added bonus of being widely available at ordinary supermarkets. Like rice bran, it's an unusually good source of fiber. It's also high in natural unsaturated oils, as rice bran is, so it's not likely to give your baked goods a drier texture. Like rice bran, it adds a pleasantly nutty flavor to your baked goods. You can emphasize the flavor by toasting your oat bran in a dry skillet and shaking it frequently, until it becomes aromatic and slightly browned. Oat bran is paler than rice bran, so it will change the appearance of your baked items slightly.
Wheat bran is also widely available in stores and can be used as a substitute in most baked goods. Wheat bran is milled into larger flakes than rice bran, which can alter the texture of your baked goods and make them coarser. Wheat bran isn't as flavorful as oat bran or rice bran, so you might need to increase your flavorings slightly. It's also less oily and more absorbent than rice or oat bran, so you might need to adapt your recipes by adding an extra tablespoon or two of oil or liquid.
Wheat bran is drier than oat bran or rice bran because, unlike those two products, its germ is separated from the bran during milling. In many recipes, wheat germ makes a better substitute for rice bran because it has a similarly high level of natural oils. Like oat bran, wheat germ is paler than rice bran and won't give the same color to baked goods. However, it's more flavorful and will often yield a better-tasting end product.
Rice bran, oat bran and wheat germ are all high in naturally-occurring oils, and that makes them perishable. Purchase them frequently in small quantities, and refrigerate to reduce the risk of their oils becoming rancid and giving off-flavors to your baking. In cookies, muffins and coffee cakes, you can use these substitutions directly. Rice bran has a finer texture than these substitutes, so for yeast-raised breads or delicate pastries it's best to process your substitute to a finer texture in a blender or spice grinder.