We mix our own. Generally, I buy a 1400# tote of 3-way (corn, oats, barley) with molasses in the winter and a separate 1400# tote of cracked corn. I use 1400# totes because that is what my tractor loader can lift out of the back of the pickup without complications. My feed man says that you cannot safely store a tote full of flaked corn because of the moisture from the steam flaking process -- it will mold even in winter. However, the 3-way mix contains flaked corn, but in that instance, the other grain absorbs the moisture from the cracked corn. As the weather warms up, we will likely cut or eliminate the molasses in the 3-way mix to eliminate the moisture and likelihood of spoilage.
When we mix rations, I keep a bucket of molasses cut with water (1 part molasses : 3 or 4 parts water) to top dress and cut down dust and to help the rumensin, mineral and other additives that we mix in. Mixing is in a 5-gallon bucket with a mortar mixer on an old electric drill. It would be nice to mix bigger batches, but this works ok for now.