VinnyT,
Not to be a smartaleck, but you only two choices of where to buy: from an individual, or from a dealer.
If you buy from an individual (I mean a non-dealer), then you have a somewhat limited selection, but you get to avoid the paperwork, transfer fees and taxes. Your state laws may restrict this process.
If you buy from a dealer, then you have nearly unlimited choices, but you may have to pay transfer fees, taxes, and background check fees. If you are buying from an online dealer, then the rifle will still have to be transferred through your local dealer with all the required paperwork. Your state laws may restrict this process as well.
Basically, look around on the 'net to decide what you like and what you are willing to pay. Then look around in your area to see what is available. That should keep you busy for quite awhile.
Have you been over to AR15.com? They have more info than you can absorb.