were can I purchase inexpensive disney pins so my kids can trade at WDW?

Almost all the Disney stores will have few pins, but they are usually at whater the regular Disney prices are.

The links below are pin dealers that usually have Disney pins at a discount. The bulk packages of 25 or so are often good deals if you are just looking for something to trade for kids – the Disney Cast members usually wear pins for trading and they are not supposed to refuse a trade from a child so long as the pin offered to trade is not already on their lanyard.

Also check to see whether there are any Pin Trading Events while you are at Disney. Some of the resorts sponsor kids-only trading events at irregular intervals.