I have acquirred and reloaded many thousands of once fired mil brass in the same calibers you mentioned. It is sold as scrap and can in some instances be purchased at near give-away prices.
I have also acquired mil brass in 38spl, 45Acp, 30-06 & 30 carb. in the past.
If you are on a base, sometimes you can purchase brass outright at the local DRMO.
I have been very satified with mil brass, but one must accept the case prep associated with it. IE, primer pocket swaging & trimming.
Also some lots in 223 are made with softer than normal brass which will result in more than your share of stuck cases.
One last note, do not be dissapointed if you find bent, stepped on or crushed cases in your lot. It is sold as scrap.