You can assemble one yourself. If you can change your own oil you have more mechanical skill than needed to put an AR15 together. You can also save some money to spend on mags and ammo.
Then get one of these. The stripped lower requires tapping of pins but I got mine built in a half hour with no previous experience. The other options are already assembled for you and all you have to do is snap an upper receiver on it and go to the range. AR-15 Lower Receivers The lower is the only part that needs to go through a dealer.
Now to select an upper. If you want to go really cheap check out Del-Ton's rifle kits. These don't really have the same level of quality control as more expensive manufacturers, but I'm sure will work just fine for plinking. M4 Rifle Kits, AR 15 Kits, Gun Kits | Del Ton Rifles from Del-Ton, Inc. If you get the kit, it comes with all the parts to assemble a stripped lower as well.
If you want an upper that has a much better following, check out what Bravo Company Machining sells. Bravo Company USA, Inc. AR-15, M16, M4 Tactical Gear, Parts, Accessories. If you get a complete lower and just order an upper from these guys don't forget to add a bolt carrier group and charging handle.
Bravo Company makes great complete rifles, or as mentioned piece one together yourself (either piece by piece or by upppers and lowers seperately, stock and grip etc...). There is a guy named Paul Lee on Mesa (in the shopping center with the Weinerschnitzel/Albertsons) that can perhaps receive the lower or stripped lower for you. the rest can be delivered to your door.