Go to http://www.ttiarmory.com/Store/
and they offer a decent selection of new match grade 50 BMG ammo. May be more then you want to pay for plinking ammo but this is good stuff. New, clean, top grade bullets and no fear of corrosive primers either. Basically as good as most handloaded ammo but not taylored to your rifle.
Probably the best buy for the money would be here: http://www.tbirdammo.com/50cal.htm
The remanufactored 660 gr or M33 ball ammo is what I would look at for a plinking round. Again, good clean fresh ammo but not what I would call Match grade. Will smack the hell out of rocks though!!! For $1.95 per round, very hard to beat for practice ammo.
I can not load same quality ammo for much less then this.
These are the two places I am personally aware of. Good ammo, clean, fresh, non corrosive. I would not shoot bulk surplus ammo in your rifle. To great a risk with bad ammo out there. You may be able to find some cheaper but you run the risk of bad ammo that can at times damage your rifle.
TO be honest, you REALLY need to save up and get some loading equipment. Does not cost as much as you would think if you get the Hornady or RCBS kits and even with those lower priced kits, your handloaded ammo will be much more accurate then any mass produced ammo.
I would not mind at all loading you some to your specs but just do not have the time anymore to do that for customers.
If you get into handloading let me know and I can get you all the data I have. One thing you will find with the AR-50 is that the throats are generally VERY long. Mine was and so were several others I have loaded for. About 1/2" longer throat then I like to see but with handloads they generally shoot extremely well still. Hell, even with standard ammo they generally shoot pretty darn well.