Where are the cheap ARs at online?

Definine " Cheap AR's", please.

Why do you want a cheap AR?

The cheapest I've ever seen an AR was ~ $650, and that was a Del-Ton kit build on a stripped Aero Precision lower with a MagPul MBUS rear sight. Shipping, FFL transfer fees included. Nothing fancy to it at all, but it worked okay.

For a complete price estimate of every single part, and where to get it, to put it into a solid, professional-quality rifle:

$400 - BCM upper receiver assembly (no fancy rails)
$90 - Stripped lower of your choice of manufacturer (most are about the same quality anyway)
$94 - G&R Tactical lower parts kit with MagPul trigger guard and stock trigger
$65 - Daniel Defense A1.5 Rear Sight
$65 - BCM M4 stock kit w/ mil-spec receiver extension
$130 - LMT Bolt Carrier group, auto
$5 - A2 grip (at least that's what they go for around here)
$45 - BCM Gunfighter charging handle
$34 - MagPul MOE midlength handguards
Free - built using the instructions found here.

That's $930 for a rifle that is a heckuva good compromise between carbine maneuverability and rifle handling and accuracy, and can handle range use, competition, carbine classes, and war that will last thousands upon thousands of rounds, and is compatible with every quality spare part made for the AR platform.

I've built both the Del-Tons mentioned above and (very nearly--the only real difference is the UBR stock) the mostly-BCM rifle listed. The Del-Tons were fun, but the BCM ran through a local carbine course of about 1100 rounds (tallied 37 magazine changes ) in two days with no cleaning, generous oil, and Federal XM193. Had zero problems even with Brown Bear 55gr, whereas my Del-Tons didn't run as reliably or as accurately.

It's quite possible to get solid, dependable quality for not a whole lot more, with a little searching... and I've kinda already done that for you.

Besides, the real cost of owning an AR is in the optics, ammo, magazines, and so forth, but mainly those three.

Yes the CMMG bargain bin has not been available for a few months now. Great ARs and hard to beat for what you get. Seems like everything I have been seeing is around $750 to $950 depending on the name.

Centerfire has some for $624.00, no idea about the quality:
http://centerfiresystems.com/ar-ia-car-gb.aspx

Spikes Tactical has quality rifles for $799.00
http://www.spikestactical.com/new/z/st15-m4-le-carbine-p-387.html

Or, build up Dionys suggestion.

I've seen the new S&W M&P15 Sport rifles for under $600 in local shops, but I've not looked for them online.

We've got S&W M&P15 ORC (optic ready carbine) rifles locally for 599$ I think the DPMS Sportical is pretty much the same thing for the same price. I know that there are a few places online that are selling for the same prices. I wouldn't rule out Stag, Rock River, and the other S&Ws. They are all good. If you want milspec, try to hunt down a Colt, BCM, or Spikes.

Just to clarify the $599 S&W is the M&P-15 Sport, which does not have a dust cover, nor forward assist; but does have an MPI BCG, detachable rear sight, correct F height A-Frame sight gas block, and a 1:8 twist 5R rifled bbl w/ Melonite finish. The M&P-15OR has no sights, does have the dust cover & forward assist; also has a 1:9 conventionally rifled chrome lined bbl w/ parkerized external finish, railed gas block, and the MPI BCG standard to the M&P-15 line.

The DPMS Panther Sportical, like the M&P-15 Sport, lacks the dust cover & forward assist - and that's where the similarities end. The Sportical has a 1:9 twist, unlined, conventionally rifled bbl. DPMS does not use an MPI BCG in the Sportical, nor most of their rifles.

If the dustcover & forward assist is important to you the new (new-ish) DPMS Panther Oracle is basically a Sportical with a conventional upper for the same price.

If the FA and DC are not big concerns, then then M&P-15 Sport has the 1:8 5R bbl & MPI BCG which are huge features in a rifle at that price.