There are at least two kinds of Sherwin Williams paint stores: House paint or Auto paint. The House paint folks might be able to order auto paint, but I doubt you'd get questions answered professionally. Dealing with an auto paint specialty supplier is recommended to get the correct products, instructional "P-sheets" and questions answered.
Since you're not far from St. Louis, contact Cooper Color with stores in Arnold, MO and Bellville, IL. They sell PPG economy to top of the line materials.
http://coopercolor.com/Tell the owner, Dave Cooper or the tech support guy, Chuck Taylor, that I sent you over to get treated right.

If you want to get serious, sign up for a body shop/paint class at your local community college in the evening this Fall. You'll learn the right way to work metal, preparation and painting. Or like Paul says, just try it on your own, following product instructions on something you can afford to mess up on.
Remember that sprayed paint always looks the best right before it runs. LOL Knowing how to get it to that edge and not too far takes the "feel" of experience.