If you Walmart sells ammo
The .243 is cheap and readily available. Around here, it usually runs $15-16 per box of 20.
You can also find Prvi partisan ammo in .243 at many of the gun supply sites for less but with the shipping comes out to about the same in quantities of less than 500 rounds. I have shot the Prvi ammo in my .243 and think it is every bit as good as the Federal ammo I purchase from Walmart.
If you ever decide to reload, the Prvi brass is great!