As stated already, so long as you are not a felon you may purchase body armor. There is a catch to this though. Since most retailers won't sell armor to you unless they know you are not a felon, you will need some kind of 'validation', usually a letter from your local leo's saying that you are not a felon. Not a huge deal, but you may go to purchase it and find out that the retailer you are purchasing from requires something like this.
That said, armor is pretty hot in the summer, and it's not the most comfortable. I wear level III most of the time, and on occasion I wear level IV. If you plan to go to level IV, but aren't sure you will want the bulk and weight of level IV all the time, I would suggest getting a level IIIA vest to wear, and add over the vest plates to go to level IV. It would be more comfortable and save you some weight most of the time. I'm not sure why level IV is thought to be illegal, but I have had no issues purchasing it in Missouri. My first vest was a III with rifle plates over it. My second vest was a IV, and I just bought some plates for another vest.
If you wanna conceal your armor, IIA is do-able. If you try to conceal IIIA-III it will look like you are concealing armor under your shirt. IIA may also be a good choice for you depending on what kind of threat you intend to stop. Just do some research on it. It's not the most comfortable thing, but it is comforting if you're in a bad spot.
It is expensive up to $1000 for some of this stuff. Express Police in KC KS has been good to deal with, as has Kellys in Springfield. BulletProofMe.com may also be a good source for you. Thats where I got plates and IV stuff.