Well, my husband is a farmer, and sorry to say, even he does not buy (most) fertilizers by the 50-pound bag... he buys them by the ton. He has the local co-op pre-mix the fertilizers for him, because that is WAY easier than buying them individually and then buying a separate piece of equipment just to mix them! Plus, imagine all of the bags lying around... the seed bags are bad enough. We don't need fertilizer bags, too!
Not all co-ops stock all fertilizers. I know that the local co-op has the most common things, but if he wants a less commonly used fertilizer, he has to travel to whichever branch of the co-op stocks that fertilizer. And, he usually does not use the same chemical formulas of fertilizers that I use in my aquarium- there are different concerns for fish tanks as for field crops. We want water soluble ferts. Farmers often prefer less readily water soluble, so that the next rain does not leach them out of the ground.
I wish you luck in your quest!! Hopefully it will be much easier for you to find these than I am imagining.