I usually go on ebay to look for lots of 10 pins.
One of the problems with those 100 pin lots of the 2000 pin. Is it's traded ALL the time. And under trading rules, you're not allowed to trade with a CM if they already have that pin on their lanyard. There's a placard on the bottom of the CM lanyard with the info on pin trading.
Out of good spirit and guest satisfaction, they can however accept non disney (Sedesma is a big one) or duplicate pins for trading. At the end of the day, these pins are taken off the lanyard and sent to the pin replenishment where they're boxed up and sent off.
It's not hard to find the cheapies on ebay, but still expect to pay a couple dollars per pin. I've got tons of trader pins that were bought for cheap when I was a CM. But I'll still buy off of ebay for harder to find pins or old ones.