I've had good luck here: http://www.earplugstore.com/foam-ear...uantities.html
Most industrial suppliers(Grainger) or safety suppliers (Lab Safety Supplies) carry bulk earplugs.
I recently got a set of custom molded siicone plugs - a guy was set up a gun show, he is some sort of audio-something or other. He takes the mold of your ear and then does some more work on them and you pick them up the same day.
I was skeptical but I've shot pistols and ARs with them now and have been really pleased. It's loud under a metal cover but I keep muffs in my bag for that or indoor.
I also find it very easy to hear people talking with them in.