Besides my local wally worlds and local gun shops, I like to use gunbot.net, they have near real time tracking of who has ammo, mags, and reloading whatnots. Select your caliber at top of page and take your pick from sellers. It lists them by price per round, how many rounds for that price, type of ammo, and who the seller is. You can just click on the link provided and bam, your taken to that sellers website where you can make purchase. Some of them are big box retailers like Cabelas, some are mom and pop places you've never heard of. Makes it easy to see who has what without actually going to all these different websites checking yourself.
Nice looking AR, post some pics of the fruits of your labor after you get it running for it's intended purpose.