Check out grain elevators or farmers supply outlets also. Some elevators sell fert also, or they can at least point you to where the farmers go to get their fert.
Be careful however and make sure that any product you pick up is approved for residential turf use.
READ LABELS. I have lost count of the number of posts from people asking questions about a product that is answered in the label. There is good info on the label and you would be well served if you make it a habit of reading, and understanding the label for every product you work with.
Additionally, some commercial grade products won't be available to you unless you have the required license to apply them commercially.
Does anything feel better then a nice healthy lawn under your bare feet? Besides the feel of money in your hand...