Keep an eye out at Walmart, I have got some good deals there in the last couple of months, and for god's sake, don't buy steel cased ammo, it might cycle with no problems, but a lot of guns will not cycle it correctly and you will end up with jams, fail to ejects and other various problems because the steel cases don't expand the way brass does. currently I have almost 10,000 rounds of brass ammo, and will be cutting about a third of them once I shoot it for my .300 blackout, then I will be reloading both, that way I can cut down on cost.
Most of the ammo I have purchased has been either PMC brand or Remington American Eagle brand. They both cycle perfectly in my 5.56/.223
Also be aware, if your gun says only .223 on the barrel, you don't want to purchase 5.56 ammo for it, the pressures are higher in the 5.56 NATO round than they are in the .223, now if your gun is marked 5.56 NATO, you can shoot both of the rounds with no problem, but again, if marked only for .223 then just get .223 ammo and you won't have any problems.
Is this the gun that you bought?
http://www.del-ton.com/DT_Sport_p/dtsport.htm
If so, it looks like it is rated for the 5.56 round