If you don't mind waiting until you get to Disney, check out the outlets when you get there. We just stocked up on traders at the Disney Outlet stores. The 2-pack of mystery pins was $1.99 and there were tons of them. At $1 each, it's an even better price than some people are paying for scrappers!!
http://www.themouseforless.com/tripplanning/travel/OutletStores.shtml
We found the pins (and other cool souvenirs) at the first two, but nothing of use at the character outlet.
This was the only trip we went to the outlets, so I'm not sure if they're always there... I wouldn't risk it if they would be my only traders. But it would be good for those of you who are waiting to get to the area to buy.