I usually have good luck finding ammo at gun s shows but from a quick search I did it looks like Aim Surplus has the lowest price for 7.65x53mm Argentine with boxes of 20 just under $15.
I've had tons of fun with my sporterized 1891. I have attached a link of a thread I started about mine.
http://thefiringline.com/forums/showthread.php?t=481975
Have fun.
BB