Go on line checking for 5.56mm ball.It was a couple years ago I bought two cases of 5.56 which figured out to be about 3.00 a box. It was Lake City Ball.If I remember right I got it from www.midsouthshooterssupply.com or www.natchezss.com .Yes it's safe to shoot 5.56mm in a 223.
See other topic posted concerning 5.56 in a 223.

I have also heard 223 is very hard to find nowdays. The same might be for the 5.56mm.It's doesn't hurt to enquire about it.